Harden check for SDK platform name on Mac OS

We now use an absolute path, to prevent picking up the wrong plutil binary.
In addition we pipe the possible stderr output of plutil and xpath to the
null device, so that the final QMAKE_MAC_PLATFORM_NAME will be empty in
case of any errors, and caught by the isEmpty() check below.

Change-Id: I8ad24bf63162a76410c2ae223dd2fc48e7886bbf
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>