rust: add a build.rs script for tests

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>