Use scons target and dependency system instead of ad-hoc options.
Now is simply a matter of naming what to build. For example:
scons libgl-xlib
scons libgl-gdi
scons graw-progs
scons llvmpipe
and so on. And there is also the possibility of scepcified subdirs, e.g.
scons src/gallium/drivers
If nothing is specified then everything will be build.
There might be some rough corners over the next days. Please bare with me.
When building more then one dri driver we would get warnings because
we where defining the same build target multiple times.
Also move all the dri scons targets related code into its own file.
