Rewrite the Makefile

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Some general patterns for the changes:

• Use user LDFLAGS and CFLAGS
• Generally allow user to pass well-known variables through the environment or on the command-line rather than editing the file and finding what they're called
• Get rid of non-standard variables like OPTS in favor of additive assignmet
• Remove repetitive code (compare %.a definition with previous foo_lib recipes)
• Correct build dependencies (e.g., it is frotz_curses.a that requires the curses defines.h; frotz itself requires that defines.h only indirectly)
• Reduce number of needless variables; it's just as easy to use sed to change all occurrences of "frotz" if the name ever changes as it would be to change $(NAME)
• Clarify recipes where possible by breaking out conditionals and such that can go elsewhere
• Make git usage less brittle by providing fall-backs; this also means that releases built from dist tarballs are more clearly marked by having GIT_BRANCH and GIT_HASH be "none"
• Remove variable definitions that make automagically handles when not passed by user, such as CC

I also removed any mention of the oss and sun audio options, since the existing checks raised an error if any option other than "ao" or "none" was passed

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I know I rather went crazy with it, but I think that these changes will make things easier in the long run, particularly as new interfaces crop up (such as SDL)

I also started working on building a libfrotz.so.$VERSION, but at the moment I'm stymied by the fact that the common and blorb headers refer to functions only defined in the front-end interface parts (which don't belong in a Z-machine reading library, of course). That project can wait 'til another day (-: