Merge #15277: contrib: Enable building in Guix containers

751549b52a9a4cd27389d807ae67f02bbb39cd7f contrib: guix: Additional clarifications re: substitutes (Carl Dong)
cd3e947f50db7cfe05c05b368c25742193729a62 contrib: guix: Various improvements. (Carl Dong)
8dff3e48a9e03299468ed3b342642f01f70da9db contrib: guix: Clarify SOURCE_DATE_EPOCH. (Carl Dong)
3e80ec3ea9691c7c89173de922a113e643fe976b contrib: Add deterministic Guix builds. (Carl Dong)

Pull request description:

  ~~**This post is kept updated as this project progresses. Use this [latest update link](https://github.com/bitcoin/bitcoin/pull/15277#issuecomment-497303718) to see what's new.**~~

  Please read the `README.md`.

  -----

  ### Guix Introduction

  This PR enables building bitcoin in Guix containers. [Guix](https://www.gnu.org/software/guix/manual/en/html_node/Features.html) is a transactional package manager much like Nix, but unlike Nix, it has more of a focus on [bootstrappability](https://www.gnu.org/software/guix/manual/en/html_node/Bootstrapping.html) and [reproducibility](https://www.gnu.org/software/guix/blog/tags/reproducible-builds/) which are attractive for security-sensitive projects like bitcoin.

  ### Guix Build Walkthrough

  Please read the `README.md`.

  [Old instructions no. 4](https://github.com/bitcoin/bitcoin/pull/15277#issuecomment-497303718)

  [Old instructions no. 3](https://github.com/bitcoin/bitcoin/pull/15277#issuecomment-493827011)

  [Old instructions no. 2](https://github.com/bitcoin/bitcoin/pull/15277#issuecomment-471658439)

  <details>
  <summary>Old instructions no. 1</summary>
  In this PR, we define a Guix [manifest](https://www.gnu.org/software/guix/manual/en/html_node/Invoking-guix-package.html#profile_002dmanifest) in `contrib/guix/manifest.scm`, which declares what packages we want in our environment.

  We can then invoke
  ```
  guix environment --manifest=contrib/guix/manifest.scm --container --pure --no-grafts --no-substitutes
  ```
  To have Guix:
  1. Build an environment containing the packages we defined in our `contrib/guix/manifest.scm` manifest from the Guix bootstrap binaries (see [bootstrappability](https://www.gnu.org/software/guix/manual/en/html_node/Bootstrapping.html) for more details).
  2. Start a container with that environment that has no network access, and no access to the host's filesystem except to the `pwd` that it was started in.
  3. Drop you into a shell in that container.

  > Note: if you don't want to wait hours for Guix to build the entire world from scratch, you can eliminate the `--no-substitutes` option to have Guix download from available binary sources. Note that this convenience doesn't necessarily compromise your security, as you can check that a package was built correctly after the fact using `guix build --check <packagename>`

  Therefore, we can perform a build of bitcoin much like in Gitian by invoking the following:

  ```
  make -C depends -j"$(nproc)" download && \
      cat contrib/guix/build.sh | guix environment --manifest=contrib/guix/manifest.scm --container --pure --no-grafts --no-substitutes
  ```

  We don't include `make -C depends -j"$(nproc)" download` inside `contrib/guix/build.sh` because `contrib/guix/build.sh` is run inside the container, which has no network access (which is a good thing).
  </details>

  ### Rationale

  I believe that this represents a substantial improvement for the "supply chain security" of bitcoin because:

  1. We no longer have to rely on Ubuntu for our build environment for our releases ([oh the horror](https://github.com/bitcoin/bitcoin/blob/72bd4ab867e3be0d8410403d9641c08288d343e3/contrib/gitian-descriptors/gitian-linux.yml#L10)), because Guix builds everything about the container, we can perform this on almost any Linux distro/system.
  2. It is now much easier to determine what trusted binaries are in our supply chain, and even make a nice visualization! (see [bootstrappability](https://www.gnu.org/software/guix/manual/en/html_node/Bootstrapping.html)).
  3. There is active effort among Guix folks to minimize the number of trusted binaries even further. OriansJ's [stage0](https://github.com/oriansj/stage0), and janneke's [Mes](https://www.gnu.org/software/mes/) all aim to achieve [reduced binary boostrap](http://joyofsource.com/reduced-binary-seed-bootstrap.html) for Guix. In fact, I believe if OriansJ gets his way, we will end up some day with only a single trusted binary: hex0 (a ~500 byte self-hosting hex assembler).

  ### Steps to Completion

  - [x] Successfully build bitcoin inside the Guix environment
  - [x] Make `check-symbols` pass
  - [x] Do the above but without nasty hacks
  - [x] Solve some of the more innocuous hacks
  - [ ] Make it cross-compile (HELP WANTED HERE)
    - [x] Linux
      - [x] x86_64-linux-gnu
      - [x] i686-linux-gnu
      - [x] aarch64-linux-gnu
      - [x] arm-linux-gnueabihf
      - [x] riscv64-linux-gnu
    - [ ] OS X
      - [ ] x86_64-apple-darwin14
    - [ ] Windows
      - [ ] x86_64-w64-mingw32
  - [ ] Maybe make importer for depends syntax
  - [ ] Document build process for future releases
  - [ ] Extra: Pin the revision of Guix that we build with with Guix [inferiors](https://www.gnu.org/software/guix/manual/en/html_node/Inferiors.html)

  ### Help Wanted

  [Old content no. 3](https://github.com/bitcoin/bitcoin/pull/15277#issuecomment-483318210)

  [Old content no. 2](https://github.com/bitcoin/bitcoin/pull/15277#issuecomment-471658439)

  <details>
  <summary>Old content no. 1</summary>
  As of now, the command described above to perform a build of bitcoin a lot like Gitian works, but fails at the `check-symbols` stage. This is because a few dynamic libraries are linked in that shouldn't be.

  Here's what `ldd src/bitcoind` looks like when built in a Guix container:
  ```
  	linux-vdso.so.1 (0x00007ffcc2d90000)
  	libdl.so.2 => /gnu/store/h90vnqw0nwd0hhm1l5dgxsdrigddfmq4-glibc-2.28/lib/libdl.so.2 (0x00007fb7eda09000)
  	librt.so.1 => /gnu/store/h90vnqw0nwd0hhm1l5dgxsdrigddfmq4-glibc-2.28/lib/librt.so.1 (0x00007fb7ed9ff000)
  	libstdc++.so.6 => /gnu/store/4sqps8dczv3g7rwbdibfz6rf5jlk7w90-gcc-5.5.0-lib/lib/libstdc++.so.6 (0x00007fb7ed87c000)
  	libpthread.so.0 => /gnu/store/h90vnqw0nwd0hhm1l5dgxsdrigddfmq4-glibc-2.28/lib/libpthread.so.0 (0x00007fb7ed85b000)
  	libm.so.6 => /gnu/store/h90vnqw0nwd0hhm1l5dgxsdrigddfmq4-glibc-2.28/lib/libm.so.6 (0x00007fb7ed6da000)
  	libgcc_s.so.1 => /gnu/store/4sqps8dczv3g7rwbdibfz6rf5jlk7w90-gcc-5.5.0-lib/lib/libgcc_s.so.1 (0x00007fb7ed6bf000)
  	libc.so.6 => /gnu/store/h90vnqw0nwd0hhm1l5dgxsdrigddfmq4-glibc-2.28/lib/libc.so.6 (0x00007fb7ed506000)
  	/gnu/store/h90vnqw0nwd0hhm1l5dgxsdrigddfmq4-glibc-2.28/lib/ld-linux-x86-64.so.2 => /usr/lib64/ld-linux-x86-64.so.2 (0x00007fb7ee3a0000)
  ```

  And here's what it looks in one of our releases:
  ```
  	linux-vdso.so.1 (0x00007ffff52cd000)
  	libpthread.so.0 => /usr/lib/libpthread.so.0 (0x00007f87726b4000)
  	librt.so.1 => /usr/lib/librt.so.1 (0x00007f87726aa000)
  	libm.so.6 => /usr/lib/libm.so.6 (0x00007f8772525000)
  	libgcc_s.so.1 => /usr/lib/libgcc_s.so.1 (0x00007f877250b000)
  	libc.so.6 => /usr/lib/libc.so.6 (0x00007f8772347000)
  	/lib64/ld-linux-x86-64.so.2 => /usr/lib64/ld-linux-x86-64.so.2 (0x00007f8773392000)
  ```

  ~~I suspect it is because my script does not apply the gitian-input patches [described in the release process](https://github.com/bitcoin/bitcoin/blob/master/doc/release-process.md#fetch-and-create-inputs-first-time-or-when-dependency-versions-change) but there is no description as to how these patches are applied.~~ It might also be something else entirely.

  Edit: It is something else. It appears that the gitian inputs are only used by [`gitian-win-signer.yml`](https://github.com/bitcoin/bitcoin/blob/d6e700e40f861ddd6743f4d13f0d6f6bc19093c2/contrib/gitian-descriptors/gitian-win-signer.yml#L14)
  </details>

  ### How to Help

  1. Install Guix on your distro either [from source](https://www.gnu.org/software/guix/manual/en/html_node/Requirements.html) or perform a [binary installation](https://www.gnu.org/software/guix/manual/en/html_node/Binary-Installation.html#Binary-Installation)
  2. Try out my branch and the command described above!

ACKs for top commit:
  MarcoFalke:
    Thanks for the replies. ACK 751549b52a9a4cd27389d807ae67f02bbb39cd7f
  laanwj:
    ACK 751549b52a9a4cd27389d807ae67f02bbb39cd7f

Tree-SHA512: 50e6ab58c6bda9a67125b6271daf7eff0ca57d0efa8941ed3cd951e5bf78b31552fc5e537b1e1bcf2d3cc918c63adf19d685aa117a0f851024dc67e697890a8d

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