Allow overriding of the libgit2 pkgconfig at build time

[ReillyBrogan]

It is often desirable to Linux distributions to de-bundle packages as much as possible so that system versions of libraries are used instead of statically compiled versions. This is problematic with libgit2-sys as the supported version range is so narrow (also a problem for most libgit2 bindings). A common solution to this is to have co-installable versions of libgit2 where the pkgconfig file is renamed (perhaps to something like libgit-1.7 or similar). Support this in libgit2-sys so that distributions can override this at build time for arbitrary applications that may use the libgit2 bindings.