Merge bitcoin/bitcoin#31741: multiprocess: Add libmultiprocess git subtree

babb9f5db6 depends: remove non-native libmultiprocess build (Cory Fields)
5d105fb8c3 depends: Switch libmultiprocess packages to use local git subtree (Ryan Ofsky)
9b35518d2f depends, moveonly: split up int_get_build_id function (Ryan Ofsky)
2d373e2707 lint: Add exclusions for libmultiprocess subtree (Ryan Ofsky)
e88ab394c1 doc: Update documentation to explain libmultiprocess subtree (Ryan Ofsky)
d4bc563982 cmake: Fix clang-tidy "no input files" errors (Ryan Ofsky)
abdf3cb645 cmake: Fix warnings from boost headers (Ryan Ofsky)
8532fcb1c3 cmake: Fix ctest mptest "Unable to find executable" errors (Ryan Ofsky)
d597ab1dee cmake: Support building with libmultiprocess subtree (Ryan Ofsky)
69f0d4adb7 scripted-diff: s/WITH_MULTIPROCESS/ENABLE_IPC/ in cmake (Ryan Ofsky)
a2f28e4be9 Squashed 'src/ipc/libmultiprocess/' content from commit 35944ffd23fa (Ryan Ofsky)
d6244f85c5 depends: Update libmultiprocess library to simplify cmake subtree build (Ryan Ofsky)

Pull request description:

  This adds the [libmultiprocess](https://github.com/chaincodelabs/libmultiprocess) library and code generator as a subtree in `src/ipc/libmultiprocess` and allows it to be built with the cmake `-DENABLE_IPC` option, which is disabled by default.

  This PR does not entirely remove the depends system [libmultiprocess package](https://github.com/bitcoin/bitcoin/blob/master/depends/packages/native_libmultiprocess.mk) because the package is useful when cross compiling. (A cross-compiling cmake build cannot easily build and run a native code generation tool.) However, it does update the depends package to build from the new git subtree, instead of being downloaded separately from github, so the same sources are used to build both the runtime library and the code generator.

  This PR includes the following manual changes (not created automatically with `git subtree add`) which just update the build system and documentation:

  - [`d6244f85c509` depends: Update libmultiprocess library to simplify cmake subtree build](d6244f85c5)
  - [`69f0d4adb72c` scripted-diff: s/WITH_MULTIPROCESS/ENABLE_IPC/ in cmake](69f0d4adb7)
  - [`d597ab1dee6b` cmake: Support building with libmultiprocess subtree](d597ab1dee)
  - [`8532fcb1c30d` cmake: Fix ctest mptest "Unable to find executable" errors](8532fcb1c3)
  - [`abdf3cb6456f` cmake: Fix warnings from boost headers](abdf3cb645)
  - [`d4bc5639829f` cmake: Fix clang-tidy "no input files" errors](d4bc563982)
  - [`e88ab394c163` doc: Update documentation to explain libmultiprocess subtree](e88ab394c1)
  - [`2d373e27071f` lint: Add exclusions for libmultiprocess subtree](2d373e2707)
  - [`9b35518d2f3f` depends, moveonly: split up int_get_build_id function](9b35518d2f)
  - [`5d105fb8c3ff` depends: Switch libmultiprocess packages to use local git subtree](5d105fb8c3)
  - [`babb9f5db641` depends: remove non-native libmultiprocess build](babb9f5db6)

  ---

  Previous minisketch subtree PR #23114 may be useful for comparison

  Instructions for subtree verification can be found:

  - https://github.com/bitcoin/bitcoin/blob/master/doc/developer-notes.md#subtrees
  - https://github.com/bitcoin/bitcoin/tree/master/test/lint#git-subtree-checksh

  TL&DR:

  ```sh
  git remote add --fetch libmultiprocess https://github.com/chaincodelabs/libmultiprocess.git
  test/lint/git-subtree-check.sh -r src/ipc/libmultiprocess
  ```

  ---

  This PR is part of the [process separation project](https://github.com/bitcoin/bitcoin/issues/28722).

ACKs for top commit:
  Sjors:
    re-ACK babb9f5db6
  TheCharlatan:
    tACK babb9f5db6
  vasild:
    ACK babb9f5db6

Tree-SHA512: 43d4eecca5aab63e55c613de935965666eaced327f9fe859a0e9c9b85f7685dc16c5c8d6e03e09ca998628c5d468633f4f743529930b037049abe8e0101e0143

doc/developer-notes.md

@@ -1335,6 +1335,9 @@ Current subtrees include:
 - src/minisketch
   - Upstream at https://github.com/bitcoin-core/minisketch ; maintained by Core contributors.
 
+- src/ipc/libmultiprocess
+  - Upstream at https://github.com/bitcoin-core/libmultiprocess ; maintained by Core contributors.
+
 Upgrading LevelDB
 ---------------------