cmake: Explicitly specify Boost_ROOT for Homebrew's package

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

Author: hebasto

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.