CMake: use GCC's -fuse-ld=mold support (mold now requires GCC 12.1)

GCC did not support mold when support was initially added,
since then GCC has been updated to add support, removing the need
to point to a binary directory containing an alternative `ld` command.

Support for MOLD_BIN CMake option has been kept as mold may be installed
to standard location.

build_files/cmake/platform/platform_unix.cmake

@@ -815,64 +815,49 @@ if(CMAKE_COMPILER_IS_GNUCC)
   string(PREPEND CMAKE_CXX_FLAGS_RELWITHDEBINFO "${GCC_EXTRA_FLAGS_RELEASE} ")
   unset(GCC_EXTRA_FLAGS_RELEASE)
 
-  # NOTE(@campbellbarton): Eventually mold will be able to use `-fuse-ld=mold`,
-  # however at the moment this only works for GCC 12.1+ (unreleased at time of writing).
-  # So a workaround is used here "-B" which points to another path to find system commands
-  # such as `ld`.
   if(WITH_LINKER_MOLD AND _IS_LINKER_DEFAULT)
     find_program(MOLD_BIN "mold")
     mark_as_advanced(MOLD_BIN)
+
     if(NOT MOLD_BIN)
       message(STATUS "The \"mold\" binary could not be found, using system linker.")
       set(WITH_LINKER_MOLD OFF)
+    elseif(CMAKE_C_COMPILER_VERSION VERSION_LESS 12.1)
+      message(STATUS "GCC 12.1 or newer is required for th MOLD linker.")
+      set(WITH_LINKER_MOLD OFF)
     else()
-      # By default mold installs the binary to:
-      # - `{PREFIX}/bin/mold` as well as a symbolic-link in...
-      # - `{PREFIX}/lib/mold/ld`.
-      # (where `PREFIX` is typically `/usr/`).
-      #
-      # This block of code finds `{PREFIX}/lib/mold` from the `mold` binary.
-      # Other methods of searching for the path could also be made to work,
-      # we could even make our own directory and symbolic-link, however it's more
-      # convenient to use the one provided by mold.
-      #
-      # Use the binary path to "mold", to find the common prefix which contains "lib/mold".
-      # The parent directory: e.g. `/usr/bin/mold` -> `/usr/bin/`.
-      get_filename_component(MOLD_PREFIX "${MOLD_BIN}" DIRECTORY)
-      # The common prefix path: e.g. `/usr/bin/` -> `/usr/` to use as a hint.
-      get_filename_component(MOLD_PREFIX "${MOLD_PREFIX}" DIRECTORY)
-      # Find `{PREFIX}/lib/mold/ld`, store the directory component (without the `ld`).
-      # Then pass `-B {PREFIX}/lib/mold` to GCC so the `ld` located there overrides the default.
-      find_path(
-        MOLD_BIN_DIR "ld"
-        HINTS "${MOLD_PREFIX}"
-        # The default path is `libexec`, Arch Linux for e.g.
-        # replaces this with `lib` so check both.
-        PATH_SUFFIXES "libexec/mold" "lib/mold" "lib64/mold"
-        NO_DEFAULT_PATH
-        NO_CACHE
+      get_filename_component(MOLD_BIN_DIR "${MOLD_BIN}" DIRECTORY)
+      # Check if the `-B` argument is required.
+      # This will happen when `MOLD_BIN` points to a non-standard location.
+      # Keep this option as mold is not yet a standard system component and
+      # users may have it installed in some unexpected place.
+      set(_mold_args "-fuse-ld=mold")
+      execute_process(
+        COMMAND ${CMAKE_C_COMPILER} -B ${MOLD_BIN_DIR} ${_mold_args} -Wl,--version
+        ERROR_QUIET OUTPUT_VARIABLE LD_VERSION_WITH_DIR
       )
-      if(NOT MOLD_BIN_DIR)
-        message(STATUS
-          "The mold linker could not find the directory containing the linker command "
-          "(typically "
-          "\"${MOLD_PREFIX}/libexec/mold/ld\") or "
-          "\"${MOLD_PREFIX}/lib/mold/ld\") using system linker."
-        )
-        set(WITH_LINKER_MOLD OFF)
+      execute_process(
+        COMMAND ${CMAKE_C_COMPILER} ${_mold_args} -Wl,--version
+        ERROR_QUIET OUTPUT_VARIABLE LD_VERSION
+      )
+      if(NOT (LD_VERSION STREQUAL LD_VERSION_WITH_DIR))
+        string(PREPEND _mold_args "-B \"${MOLD_BIN_DIR}\" ")
+        set(LD_VERSION "${LD_VERSION_WITH_DIR}")
       endif()
-      unset(MOLD_PREFIX)
-    endif()
 
-    if(WITH_LINKER_MOLD)
-      # GCC will search for `ld` in this directory first.
-      string(APPEND CMAKE_EXE_LINKER_FLAGS    " -B \"${MOLD_BIN_DIR}\"")
-      string(APPEND CMAKE_SHARED_LINKER_FLAGS " -B \"${MOLD_BIN_DIR}\"")
-      string(APPEND CMAKE_MODULE_LINKER_FLAGS " -B \"${MOLD_BIN_DIR}\"")
-      set(_IS_LINKER_DEFAULT OFF)
+      if("${LD_VERSION}" MATCHES "mold ")
+        string(APPEND CMAKE_EXE_LINKER_FLAGS    " ${_mold_args}")
+        string(APPEND CMAKE_SHARED_LINKER_FLAGS " ${_mold_args}")
+        string(APPEND CMAKE_MODULE_LINKER_FLAGS " ${_mold_args}")
+        set(_IS_LINKER_DEFAULT OFF)
+      else()
+        message(STATUS "GNU mold linker isn't available, using the default system linker.")
+      endif()
+      unset(_mold_args)
+      unset(MOLD_BIN_DIR)
+      unset(LD_VERSION)
     endif()
     unset(MOLD_BIN)
-    unset(MOLD_BIN_DIR)
   endif()
 
   if(WITH_LINKER_GOLD AND _IS_LINKER_DEFAULT)