crabtime provides crabtime::WORKSPACE_PATH to refer to the
CARGO_MANIFEST_DIR of the top level crate being built, which means when
building libcosmic directly, crabtime::WORKSPACE_PATH will work, but
when building it as a dependency of another crate,
crabtime::WORKSPACE_PATH will no longer refer to the path to libcosmic.
I don't think there's a good workaround, since when in the context of
crabtime, CARGO_MANIFEST_DIR refers to the path to the crate generated
by crabtime rather than to libcosmic.
This replaces crabtime with a simple build.rs script that generates a
file in OUT_DIR.
