Change test for GIT_VERSION_CMD an xcode check

Following the work I did on GH ' Remove special casing on GIT_VERSION_CMD #33 '
I've done more experiements and test rebuilds. I've found that the test should
actually be to find out if we are working with Xcode specifically as using the
gnu build chain is working correctly with the code in the 'else' clause - the
codepath i tested before doing my original changes - but doesn't work using the
special 'apple' version.

This change should mean both methods of building work correctly.
Merge pull request #37 from goetzk/change-git-version-cmd-test

Change test for GIT_VERSION_CMD an xcode check