allow modern GCC to be automatically chosen on OS X 10.5

Don't force gcc-4.2 for OS X SDK 10.5 when $CC version is newer than 4.2

Change-Id: I7031517f8c2140a9dcb56abe6f6db5c9ed8f21f9
Reviewed-on: https://gerrit.libreoffice.org/10118
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>