Issue: 920.  Do not enforce gcc compiler on Apple platforms for CMake builds.

git-svn-id: svn://dev.exiv2.org/svn@3172 b7c8b350-86e7-0310-a4b4-de8f6a8f16a3
Tweaked Makefile to prevent re-linking the exiv2 binary unnecessarily.

git-svn-id: svn://dev.exiv2.org/svn@3173 b7c8b350-86e7-0310-a4b4-de8f6a8f16a3