Support running a sanity check

[lorin]

A naive user might try to use MixDown to do a build without having installed some essential tooling such as gcc or libtool. It would be nice if MixDown could do a sanity check to see if all the required tooling was installed.

It could be either invoked manually (e.g., using a --sanity-check flag), or maybe done automatically before attempting to do a build. If done automatically, it could do a check only if libtool is needed.

[lorin]

Example of something to sanity-check for: python development files (e.g., Python.h). This is called something like python-dev or python-devel on ubuntu.