Find correct include paths for C library parsing

[mell-o-tron]

Currently, function get_lib_path in file lib/typecheck.ml uses "/usr/include/..." as the fixed path to include libraries.

Fix this, figuring out what the include paths are.

• Maybe use something like echo | gcc -E -x c - -v to figure out where gcc gets the includes
• Maybe add a way to manually specify where to find a library

If both are implemented, give priority to manually specified paths.