ghc --make + #include directories

[George Russell]

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.