rust: add a build.rs script for tests

Cargo uses the build.rs script to determine how to compile and link a binary. The only binary we're generating, however, is for our tests, but in a future commit, we're going to link against libgit.a for some functionality and we'll need to make sure the test binaries are complete. Add a build.rs file for this case and specify the files we're going to be linking against. Because we cannot specify different dependencies when building our static library versus our tests, update the Makefile to specify these dependencies for our static library to avoid race conditions during build. Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>