Allow Rugged to be built using CMake 2.6.

CMake 2.6 does not support the `--build` flag, but instead supports
building through GNU Make just fine. This is also compatible with all
newer CMake versions.
Merge pull request #328 from libgit2/arthur/bring-back-cmake-2.6-support

Allow Rugged to be built using CMake 2.6.