Automatically re-generate Makefile when Makefile.in changed

This avoids having an out of date copy of the Makefile when it source,
Makefile.in, changed.

The implementation here only works with GNU Make, but it should not
have undesirable effects with other Make implementations (if we ever
support them).
Merge pull request #185 from b4n/make/auto-makefile-regen

Automatically re-generate Makefile when Makefile.in changed