Cabal C include search paths

[TravisCardwell]

We would like to provide an option to get C include search path information from a section of a .cabal file. The file should of course be parsed using the Cabal library.

• Any directories specified in include-dirs should be resolved and added to the non-system include search path.
• For packages specified in pkgconfig-depends, we can call pkg-config and add to the include search path according to the result.

This was discussed in #366 (comment)