Remove special casing on GIT_VERSION_CMD

This special casing has been causing cmake -GXcode to fail.
Re-add IF check around GIT_VERSION_CMD

Previous removal included a APPLE check which I'm told needs to remain.
Reintroduce ELSE clause

Had a chat with softcoder who explained what was going on. We should now be
almost identical to the original but the compiler specific test has been
removed.
Merge pull request #33 from goetzk/git-version-cmd-fix

Remove special casing on GIT_VERSION_CMD