Makefile: Don't override PKG_CONFIG_PATH
In order to link git2go against our self-compiled copy of libgit2 we override the PKG_CONFIG_PATH so that it includes the libgit2.pc file in its search path. This breaks systems which have libraries installed in non-standard locations, as such systems typically set up PKG_CONFIG_PATH to make these non-standard locations discoverable.
Fix this by prepending libgit2 to PKG_CONFIG_PATH instead of overriding it.
Changelog: fixed