Xcode: Convert maybe unversioned OSX sysroot into versioned SDK path

Starting with Xcode 8 the SDK folder also contains an unversioned
entry:

$ ls -l /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs

drwxr-xr-x  5 root  wheel  170 Mar  4  2018 MacOSX.sdk
lrwxr-xr-x  1 root  wheel   10 Sep 16 20:49 MacOSX10.12.sdk -> MacOSX.sdk

If this unversioned path is used CMake cannot detect the SDK
version. To work around the problem we always invoke the code path
that translates short SDK names like "macosx10.12" into a path.
That way we always end up with a versioned SDK path in
_CMAKE_OSX_SYSROOT_PATH which is later used to determine the version.

Closes: #16323

Modules/Platform/Darwin-Initialize.cmake

@@ -125,8 +125,10 @@ if(CMAKE_OSX_SYSROOT)
       set(_CMAKE_OSX_SYSROOT_ORIG "")
     endif()
     set(_CMAKE_OSX_SYSROOT_PATH "${CMAKE_OSX_SYSROOT}")
-  else()
-    # Transform the sdk name into a path.
+  endif()
+
+  if(CMAKE_OSX_SYSROOT)
+    # Transform the (maybe unversioned) sysroot into a versioned path.
     execute_process(
       COMMAND xcodebuild -sdk ${CMAKE_OSX_SYSROOT} -version Path
       OUTPUT_VARIABLE _stdout