Fix LTO with GCC 4.9

For LTO to work, GCC 4.9 requires using the wrappers gcc-ar and gcc-ranlibs:
http://gcc.gnu.org/gcc-4.9/changes.html

CMake doesn't account for this yet:
http://public.kitware.com/pipermail/cmake-developers/2014-January/009344.html
Merge pull request #337 from Tilka/lto

Fix LTO with GCC 4.9