Method _fix_compile_args

Typecheck and fix-up some of the arguments to the 'compile()' method, and return fixed-up values. Specifically: if 'output_dir' is None, replaces it with 'self.output_dir'; ensures that 'macros' is a list, and augments it with 'self.macros'; ensures that 'include_dir

(self, output_dir, macros, include_dirs)