We don't really expect any other compiler on macOS. The GCC shipped with Xcode is just an alias to. clang.

I am being a bit selfish here. I have Xcode 10.2 which is shipped with AppleClang 10.1 which doesn't support this flag. But I compiled LLVM toolchain with clang 12 and made some changes to have Xcode use clang 12.
It works fine for compiling, linking etc. But those menu items are somehow hardcoded to use AppleClang 10.

I'll make changes to not add those flags on (< Xcode 11) and make some changes locally to my initial cache file to add those complier flags.

The single line had become too long. Hope this is correct.

This can be marked as advanced.

Version checks should be paired with the compiler they're checking the version of.

The white-space is being added in the command line, this can be done instead.

    set(PLATFORM_CFLAGS
      "${PLATFORM_CFLAGS} \
-ffile-prefix-map=\"${CMAKE_SOURCE_DIR}/\"=\"\" \
-ffile-prefix-map=\"${CMAKE_BINARY_DIR}/\"=\"\"")

I would make this logic purely depend on the compiler version, not Xcode version.

(CMAKE_COMPILER_IS_GNUCC AND CMAKE_C_COMPILER_VERSION VERSION_GREATER_EQUAL 8.4) OR
(CMAKE_C_COMPILER_ID EQUAL "Clang" AND CMAKE_C_COMPILER_VERSION VERSION_GREATER_EQUAL 10.0) OR
(CMAKE_C_COMPILER_ID EQUAL "AppleClang" AND CMAKE_C_COMPILER_VERSION VERSION_GREATER_EQUAL 11.0)

Also, it's inconsistent that for Xcode it shows a warning message when unsupported, while for Linux it silently ignores.

You might as well check the compiler versions earlier and if not supported, simply don't expose the option.

From an earlier version of this patch the issue seems to be XCode not properly handling the paths. (as in - it breaks some XCode features, while the compiler works).

if(XCODE AND ${XCODE_VERSION} VERSION_LESS 11.0)
  message_first_run(WARNING "Some actions under Product menu like assembling or preprocessing"
                            " individual files may not work due to -ffile-prefix-map.")
endif()

I don't have strong opinions on this, It just seemed like the kind of thing developers could spend a long time looking into only to realize they they missed this this message, so disabling makes more sense so as not to trip developers up.

To clarify, that would happen in a case where Xcode say 10.2 is being used with a new toolchain containing llvm clang 10 or 11 or 12 etc.
The condition (CMAKE_C_COMPILER_ID EQUAL "Clang" AND CMAKE_C_COMPILER_VERSION VERSION_GREATER_EQUAL 10.0) will be met, flag will be passed and then some menu items will break.
Though I expect the person doing that much to be able to navigate around Xcode and see what the issue is in build logs (:

On Linux this is giving me an error.

CMake Error at CMakeLists.txt:1671 (if):
  if given arguments:

    "XCODE" "AND" "VERSION_LESS" "12.0"

  Unknown arguments specified

Check APPLE before the version of xcode.

if(APPLE)
  if((DEFINED XCODE) AND ...)
  endif()
endif()

Then there is the issue of this being disabled or not, suggest to disable so developers don't loose time investigating broken features, since we can't rely on them to see this message, but I'm not an XCode user.

I'd prefer if(APPLE) wrap the entire block, it means non-apple devs can ignore the contents entirely and avoids changes to the if-check impacting other platforms.