Because of the fact that pkg-config is pants-on-head retarded and that
the Linux linker *requires* a static library to come before all its
dynamic dependencies in the link path, calling `pkg-config --libs
--static` was generating the wrong flags for linking.
By setting the "Libs" line before all other rules, we make sure that
`-lgit2` is the first library in the link path and that it gets its
symbols resolved with the libraries coming after it.
This fix (ab)uses an implementation detail in `pkg-config` (namely, that
flags are output as they are found on the file), but this detail seems
to be stable between releases and always gives a stable output.