Added gecode build script for Jenkins

 - Use fake_root
 - Skip downloading source file if it already exists
 - make distclean
 - Remove the pre-existing gecode deb if it exists
Build solver with Makefile

To build solver now, at the top-level depsolver directory:

  make solver

  - `make clean` will delete the solver binaries and object files
  - `make compile` will make solver