Under --slow, the DPH tests can take incredible amounts of time to run.
And on some platforms, we may not have a fully working linker in order
to build DPH etc anyway. So this provides a way to turn it off.

This flag is off by default, but should your specify it, you can skip
the required 'dph' libraries in a build, should you remove them from the
source tree.

Signed-off-by: Austin Seipp <aseipp@pobox.com>