Building against system libs

This allows for building against "system" copies of libraries. It currently only applies to libmseed and evalresp. I only really care about this for packaging purposes, so it's not super flexible (aka, all or nothing!)

The only thing I'm unsure of, is whether it works without `setuptools`. The NumPy package here depends on setuptools, so I can't get away from it to test things.

I also kind of wish IRIS would make a real release of evalresp with all of our changes, so I don't have to carry around 18 patches, but that's a different matter...