Cabal ignores c-sources when compiling modules with signatures

Basically what it says above. If you try to build a package that exports signatures *and* has some C source to compile, the latter gets completely ignored. Interestingly, it doesn't even do file existence checks.

The simplest way to see this is to just add `c-sources: applejack` to the end of the str-sig package's cabal file. This gets ignored, and the package compiles normally.

I've been testing this against cabal 2.0 head, or `5a7da47c`.