Revert "Xcode: Convert maybe unversioned OSX sysroot into versioned SDK path"

Revert commit v3.7.0-rc1~48^2 (Xcode: Convert maybe unversioned OSX
sysroot into versioned SDK path, 2016-09-25).  The replacement of
`else()` with `if(CMAKE_OSX_SYSROOT)` defeats the prior handling of
`if("x${CMAKE_OSX_SYSROOT}" MATCHES "/")`. This causes the combination

    -DCMAKE_OSX_DEPLOYMENT_TARGET:STRING="" -DCMAKE_OSX_SYSROOT:STRING=/

to not be honored and `-isysroot` to be emitted as a compiler flag
universally.  We will need another solution to the problem the
now-reverted commit was meant to address.

Closes: #16394

Modules/Platform/Darwin-Initialize.cmake

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