Make Cabal aware of pkg-config package versions.

A setup hook for Cabal that determines the compile-time version of a pkg-config package and adds CPP macros and enables flags.

A Cabal setup hook for pkg-config version info

Cabal doesn't natively give access to version information from pkg-config (yet?). This setup hook gets the version and makes it available via Haskell CPP and C/C++ macros, as well as setting Cabal flags automatically.