ghc --make + #include directories

Thanks to ghc --make it is now common to compile files from a different directory. This creates a new problem, in that if a file starts with a {-# include <c-header.h> #-} directive, ghc will try to find c-header.h in the current directory, not in the directory containing the source file. So we need a way of changing the gcc include path during compilation of .hc files to include the directory containing the source file.