[PATCH] Overhauling the buildsystem

[Thiago Macieira]

This patch series overhauls the Makefile. The first 6 patches are just
moving things around in the Makefile and changing the way we use some
variables. Then in patches 7 and 8, the Makefile is split in 3.

  Configure.mk: contains detection rules for Subsurface's dependencies
  Makefile: contains the declaration of source files and libraries
  Rules.mk: contains the rules for building

The rest of the commits are self-explanatory.