Who knows how to perform a DEBUG build using Scons? Scons is the only way i can successfully build blender.

Either set it in your config file (all sorts of debuggy goodness in there!) or do
scons BF_DEBUG=1
on the command line.

Thanks to stiv's hint about GPU_state_init(), the problem was somehow fixed by experimenting with different drivers. Only the latest radeon drivers from http://ppa.launchpad.net/xorg-edgers/ppa/ubuntu made blender work on my radeon7500.