Merge bitcoin/bitcoin#32814: cmake: Explicitly specify Boost_ROOT for Homebrew's package

8800b5a cmake: Explicitly specify `Boost_ROOT` for Homebrew's package (Hennadii Stepanov)

Pull request description:

  On macOS, this PR ensures that the Boost package is located at its real path rather than via the symlink in the default prefix.

  A backport to 29.x is required for bitcoin/bitcoin#32804, as this change prevents contamination of include directories by broad locations such as `/usr/local/include` or `/opt/homebrew/include`, which take precedence over Qt’s `-iframework` flags.

  Below is the relevant change in the configuration logs on my macOS 15.5 `x64`:
  - master branch @ ead4468:
  ```
  % cmake -B build
  <snip>
  -- Found Boost: /usr/local/include (found suitable version "1.88.0", minimum required is "1.73.0")
  <snip>
  ```
  - this PR:
  ```
  % cmake -B build
  <snip>
  -- Found Boost: /usr/local/opt/boost/include (found suitable version "1.88.0", minimum required is "1.73.0")
  <snip>
  ```

  This PR is forward-compatible with the changes proposed in bitcoin/bitcoin#32667.

ACKs for top commit:
  fanquake:
    ACK 8800b5a Checked that this plus #32805 fixes #31009

Tree-SHA512: 114bd945ec0c06a8d15b565e5b9aafc3bcfdf2a4ba4400e072b8e31053dff0f9ef423b941ee1dff2113f83e08f7fada728383ae88b3ec380b5c3e40553205f7d

Author: fanquake

cmake/module/AddBoostIfNeeded.cmake

@@ -17,6 +17,18 @@ function(add_boost_if_needed)
   directory and other added INTERFACE properties.
   ]=]
 
+  if(CMAKE_HOST_APPLE)
+    find_program(HOMEBREW_EXECUTABLE brew)
+    if(HOMEBREW_EXECUTABLE)
+      execute_process(
+        COMMAND ${HOMEBREW_EXECUTABLE} --prefix boost
+        OUTPUT_VARIABLE Boost_ROOT
+        ERROR_QUIET
+        OUTPUT_STRIP_TRAILING_WHITESPACE
+      )
+    endif()
+  endif()
+
   # We cannot rely on find_package(Boost ...) to work properly without
   # Boost_NO_BOOST_CMAKE set until we require a more recent Boost because
   # upstream did not ship proper CMake files until 1.82.0.