Have cabal_macros.h generate PKGCONFIG_VERSION and MIN_PKGCONFIG_VERSION

I ran into an issue today where I needed to write some code like this:

```
#if /* version is greater than 2.1 */
wibble
#else
wobble
#endif
```

This is in a project where `` is detected with `pkgconfig-depends`. I would have hoped that the autogenerated `cabal_macros.h` would provide some way of detecting ``'s version (as specified in `.pc`, but sadly that doesn't seem to be the case, so I have to work around it manually by using `Cabal` flags and `cpp-options: -DC_LIBRARY_VERSION=xyz`.

Would it be unreasonable to have `cabal_macros.h` spit out `PKGCONFIG_VERSION` and `MIN_PKGCONFIG_VERSION` macros for every `pkg-config` dependency? There's the issue that `pkg-config` names can contain characters that aren't permissible in `Cabal` package names. For example, `glib-2.0` contains a period. But maybe we could just turn periods into underscores like we currently do hyphens.