Patch from Khoo Yit Phang.

Patch from Khoo Yit Phang.

General clean-up of the build system.
- Remove unused or unnecessary flags and rules.
- Fix up automatic dependency generation.
- Move final linking of bin/stp to the topmost Makefile for better dependency control.
- Update versionString.cpp only if the version changed, to avoid unnecessary rebuild.
- Build src/sat/libminisat.a and lib/libstp.a by concatenating their constituent archives, rather than by relisting their contained object files.
- Add dependencies for bin/stp and lib/libstp.a to rebuild them only when necessary.

git-svn-id: https://stp-fast-prover.svn.sourceforge.net/svnroot/stp-fast-prover/trunk/stp@1126 e59a4935-1847-0410-ae03-e826735625c1