Fix typo related to CONTEXT_TYPE_HEADER

It turns out I forget to set headers to CONTEXT_TYPE_HEADER, but for
some reason was instead setting them to CONTEXT_TYPE_LIBRARY.  Whoops!
Add pkg-config support

This lets me _build_ *.pc files.  A pkg-config file is included for
pinclude, which should serve as an example of how to go about doing
that.