This addresses part of #713, by listing and building C libraries as
dependencies. This means we automatically build CLAPACK when needed, and
`lxml`'s C dependencies are not built if lxml is not. In particular,
building "core" should now be faster.
The building itself is still performed by Makefile.
* Make mkpkg command more prominent in the docs
Under "adding new packages", information on the mkpkg command is
buried under "meta.yaml". To make it easier to find,
give this command its own section.
* Update docs/new_packages.md
Co-Authored-By: Michael Droettboom <mdboom@gmail.com>