Correctly set up C(lang) preprocessor environment

[edsko]

When processing the input header files, we must ensure that any context that the preprocessor requires (paths, #defines, etc.) should all be set up correctly, ideally well integrated with the general build process (cabal, pkg-config, etc.)

[edsko]

See also #72 for the possibility of not knowing the values of certain #defines.

[edsko]

One example here is provided by xxHash, where you can define a bunch of CPP macros to configure the library. See also #90 .

[edsko]

This should be done both in bindgen-as-preprocessor as well as bindgen-in-TH mode.