Explicitly specify the language for c2ffi as c

[nagy]

I had to make this change in order to get raylib.h from www.raylib.com running. If we only support c, which I assume, then this should not break anything else. Also I changed the -o to --output because in scripts and programs I like to use the long forms of program options since it makes reading the code easier for people unfamiliar with c2ffis options.

Also in order to figure this out I had to run the program arguments manually, which were given to me as an error. The helping clue was given by c2ffi via stderr, which was not printed. It would be great if we could also print out the content of stderr if c2ffi fails to help in quicker debugging.