Merge #15584: build: disable BIP70 support by default

e09913f1c47e693b0c6fafef55b9ca78e5f3abc0 doc: specify protobuf as optional in build docs (fanquake)
376f4929f8f75011b72b2f9c3164980db482278a build: disable BIP70 support by default (fanquake)

Pull request description:

  Disable BIP70 support in the GUI by default for `0.19.0` (for eventual removal in `0.20.0`?).

  Users who want to compile with BIP70 support enabled can pass `--enable-bip70` to `./configure`.

  I've inverted the current `--disable-bip70` test to instead pass `--enable-bip70`.

  Tested configurations on `macOS` (`protobuf` installed with `brew`).
  Protobuf available and `./configure`:
  ```
  Options used to compile and link:
    with wallet   = yes
    with gui / qt = yes
      with bip70  = no
  ```

  Protobuf available and `./configure --enable-bip70`:
  ```
  Options used to compile and link:
    with wallet   = yes
    with gui / qt = yes
      with bip70  = yes
  ```

  Protobuf not available (i.e `brew unlink protobuf`) and `./configure`:
  ```
  Options used to compile and link:
    with wallet   = yes
    with gui / qt = yes
      with bip70  = no
  ```

  Protobuf not available and `./configure --enable-bip70`:
  ```
  checking whether to build test_bitcoin-qt... yes
  checking whether to build BIP70 support... configure: error: protobuf missing
  ```

  TODO:
  - [x] Remove `protobuf` from other Travis builds
  - [ ] Documentation updates (mention that `protobuf` is now optional)?
  - [ ] Could split release notes into GUI and build

ACKs for top commit:
  laanwj:
    ACK e09913f1c47e693b0c6fafef55b9ca78e5f3abc0
  elichai:
    ACK e09913f1c47e693b0c6fafef55b9ca78e5f3abc0 Read the autotools changes. awesome that this removes the protobuf requirement.
  practicalswift:
    ACK e09913f1c47e693b0c6fafef55b9ca78e5f3abc0 -- diff looks correct

Tree-SHA512: 7bf87ae8555e24db2da2e89cc4d4e90d09be27499ad386ad65879d05df8f96d9a1384379891ac8963d17728c90e55961560813df97e849e631e2de8c08e210c8

3 files changed, 15 insertions, 3 deletions

diff --git a/doc/build-osx.md b/doc/build-osx.md
index 65cfce6b15..9942449bf6 100644
--- a/doc/build-osx.md
+++ b/doc/build-osx.md
@@ -19,7 +19,7 @@ Then install [Homebrew](https://brew.sh).
 
 ## Dependencies
 ```shell
-brew install automake berkeley-db4 libtool boost miniupnpc openssl pkg-config protobuf python qt libevent qrencode
+brew install automake berkeley-db4 libtool boost miniupnpc openssl pkg-config python qt libevent qrencode
 ```
 
 See [dependencies.md](dependencies.md) for a complete overview.
diff --git a/doc/build-unix.md b/doc/build-unix.md
index eb88aca050..8f6c702f5c 100644
--- a/doc/build-unix.md
+++ b/doc/build-unix.md
@@ -112,12 +112,16 @@ To build without GUI pass `--without-gui`.
 
 To build with Qt 5 you need the following:
 
-    sudo apt-get install libqt5gui5 libqt5core5a libqt5dbus5 qttools5-dev qttools5-dev-tools libprotobuf-dev protobuf-compiler
+    sudo apt-get install libqt5gui5 libqt5core5a libqt5dbus5 qttools5-dev qttools5-dev-tools
 
 libqrencode (optional) can be installed with:
 
     sudo apt-get install libqrencode-dev
 
+protobuf (optional) can be installed with:
+
+    sudo apt-get install libprotobuf-dev protobuf-compiler
+
 Once these are installed, they will be found by configure and a bitcoin-qt executable will be
 built by default.
 
@@ -140,12 +144,16 @@ ZMQ dependencies (provides ZMQ API):
 
 To build with Qt 5 you need the following:
 
-    sudo dnf install qt5-qttools-devel qt5-qtbase-devel protobuf-devel
+    sudo dnf install qt5-qttools-devel qt5-qtbase-devel
 
 libqrencode (optional) can be installed with:
 
     sudo dnf install qrencode-devel
 
+protobuf (optional) can be installed with:
+
+    sudo dnf install protobuf-devel
+
 Notes
 -----
 The release is built with GCC and then "strip bitcoind" to strip the debug
diff --git a/doc/release-notes-15584.md b/doc/release-notes-15584.md
new file mode 100644
index 0000000000..3d9b1cc903
--- /dev/null
+++ b/doc/release-notes-15584.md
@@ -0,0 +1,4 @@
+GUI Changes
+-----------
+- In 0.18.0 a `./configure` flag was introduced to allow disabling BIP70 support in the GUI (support was enabled by default). In 0.19.0 this flag is now __disabled__ by default.
+- If you want compile Bitcoin Core with BIP70 support in the GUI, you can pass `--enable-bip70` to `./configure`.
\ No newline at end of file