In other words, don't build src/gallium source code from within src/mesa/Makefile.
Also, allow to customize which gallium auxiliary dirs, driver driver, winsys
dirs get built from the config/* files.
Update the Makefiles and includes for the new paths.
Note that there hasn't been no separation of the Makefiles yet, and make is
jumping all over the place. That will be taken care shortly. But for now, make
should work. It was tested with linux and linux-dri. Linux-cell and linux-llvm
might require some minor tweaks.
