Be explicit about dependencies

Especially when dealing with the static library, the users need to know what to link to. Make use of the existing pkg-config file we have.

---

In order to use this from a submodule/subdir/whatever, you can do

    pkg-config --libs --static vendor/libgit2/libgit2.pc

which is generated after each CMake run.

/cc @arthurschreiber