Added 'expand_makefile_vars()' to (duh) expand make-style variables

in a string (gives you something to do with the dictionary returned
  by 'parse_makefile()').
Pulled the regexes in 'parse_makefile()' out -- they're now globals,
  as 'expand_makefile_vars()' needs (two of) them.
Cosmetic tweaks to 'parse_makefile()'.