macOS: Default to modern linker on x64

This patch changes the default macOS x64 linker from the slower legacy
"classic" linker, to the much faster default modern macOS linker. A new
CMake option `WITH_LEGACY_MACOS_X64_LINKER` was also introduced to use
the old linker, set to OFF by default.

As an example from testing on an Intel Macbook, this makes linking time
go from 3min25s to 10s after this change. The reason why the legacy
linker was enforced on x64 in the first place was to silence "platform
load command not found" warnings during linking. Since this behavior is
still desired on the BuildBot, the `WITH_LEGACY_MACOS_X64_LINKER` option
is enforced in the BuildBot macOS configs.

Pull Request: https://projects.blender.org/blender/blender/pulls/134639

CMakeLists.txt

@@ -1259,6 +1259,10 @@ if(APPLE)
   include(platform_apple_xcode)
 endif()
 
+if(APPLE)
+  option(WITH_LEGACY_MACOS_X64_LINKER "Use legacy macOS linker. Only on x64 platform. Minutes slower, but emits fewer warnings." OFF)
+  mark_as_advanced(WITH_LEGACY_MACOS_X64_LINKER)
+endif()
 
 # -----------------------------------------------------------------------------
 # Check for Conflicting/Unsupported Configurations

build_files/buildbot/config/blender_macos.cmake

@@ -5,3 +5,4 @@
 include("${CMAKE_CURRENT_LIST_DIR}/../../cmake/config/blender_release.cmake")
 
 set(WITH_CYCLES_TEST_OSL     ON CACHE BOOL "" FORCE)
+set(WITH_LEGACY_MACOS_X64_LINKER ON CACHE BOOL "" FORCE)

build_files/cmake/platform/platform_apple.cmake

@@ -427,8 +427,10 @@ string(APPEND PLATFORM_LINKFLAGS
 )
 
 if(${XCODE_VERSION} VERSION_GREATER_EQUAL 15.0)
-  if("${CMAKE_OSX_ARCHITECTURES}" STREQUAL "x86_64")
+  if("${CMAKE_OSX_ARCHITECTURES}" STREQUAL "x86_64" AND WITH_LEGACY_MACOS_X64_LINKER)
     # Silence "no platform load command found in <static library>, assuming: macOS".
+    #
+    # NOTE: Using ld_classic costs minutes of extra linking time.
     string(APPEND PLATFORM_LINKFLAGS " -Wl,-ld_classic")
   else()
     # Silence "ld: warning: ignoring duplicate libraries".