Build system fixes

These are some simple changes meant to enable better use of the build system.

  * The first change adds a rule to `Makefile.in` that will re-run configure (via `config.status` to ensure the same user-options) if Makefile.in changes -- regenerating the Makefile.
  * The second change/commit enables building ChaNGa in a separate directory (e.g. by running "/path/to/changa-source/configure [options]" from that directory).  This can be useful for a number of reasons, not the least of which is the ability to have multiple different builds of the project at the same time and without much effort by the user.

I'm not 100% sure I wrote the out-of-source pattern rules in `Makefile.in` (l. 144-155) correctly, so let me know what needs changing and I'll take care of it.