Merge #18820: build: Propagate well-known vars into depends

f0d7ed10b48a6303d8b0cb6f2fc6b8652945bffb depends: Propagate only specific CLI variables to sub-makes (Carl Dong)
0a33803f1c42c938cc7c6c5291ef1f9a1dfb491b depends: boost: Use clang toolset if clang in CXX (Carl Dong)
1ce74bcde341bbab538937544a0e4b4abccdc050 depends: boost: Split target-os from toolset (Carl Dong)
2d4e48081382a58033ed5c3734a4ad810b56a963 depends: boost: Specify toolset to bootstrap.sh (Carl Dong)
3d6603e340d6d461832f0aa204b04343d34af3d4 depends: Propagate well-known vars into depends (Carl Dong)

Pull request description:

  From: https://github.com/bitcoin/bitcoin/pull/18308#issuecomment-598301117

  The following monstrosity is quite useful when invoked inside `depends`, and reviewers can use it to compare the behaviour of this change against master.
  ```bash
  make print-{{,{host,{,{i686,x86_64,riscv64}_}linux}_}{CC,CXX},boost_{cc,cxx}}
  ```

  It would also be helpful to make sure that setting `HOST`, `CC`, and `CXX` does the right thing. The 3 hosts I found offered good coverage were: `{x86_64,i686,riscv64}-linux-gnu`. As we special-case the `x86_64` and `i686` hosts in `depends/hosts/linux.mk`, and `riscv64` is a sanity check for a non-special-cased host.

ACKs for top commit:
  hebasto:
    ACK f0d7ed10b48a6303d8b0cb6f2fc6b8652945bffb, tested on Linux Mint 19.3 (x86_64):
  practicalswift:
    ACK f0d7ed10b48a6303d8b0cb6f2fc6b8652945bffb -- patch looks correct
  laanwj:
    Code review and concept ACK f0d7ed10b48a6303d8b0cb6f2fc6b8652945bffb
  ryanofsky:
    Code review ACK f0d7ed10b48a6303d8b0cb6f2fc6b8652945bffb. Changes since last review: adding comment explaining check for predefined make variables, dropping freetype commit, adding commit whitelisting overrides for recursive makes

Tree-SHA512: b6b8e76f713c26a0add6cd685824e2f5639109236ee9f89338f7c79cb1b1f2c3897bfb62b80b023d6d1943b5a6eb282a2f827f1f499c5e556eca015d6635fa65

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