Preprocessor execution mode

[edsko]

It should be possible to run the tool in preprocessor mode, generating files which can then be distributed. This avoids the need for libclang on deployment machines.

Things to consider:

• It needs to be possible to tell the preprocessor which architecture to target for cross-compilation.
• It needs to fit in the build process; we should think about what the proper workflow there is (with modern cabal?), supporting things like pkg-config (through cabal), etc. (Correctly set up C(lang) preprocessor environment #71)

[edsko]

Closing; we can specify build target through clang arguments, and #71 covers the second point.