Linux sources compile for real now

Turns out I was using a stale version of libosmesa.so during my last
commit, and the new libosmesa doesn't actually build.

This CL:
1. Reverts an unnecessary change to querymatrix.c. Instead, I'm opting
for defining _GNU_SOURCE instead, as the mesa project does.
2. Modifies the set of sources to make sure all necessary symbols get
defined (so dlopen() succeeds)
3. Creates a new target which runs the source generation step that mesa
performs.

As of now, osmesa builds and dlopen() succeeds. However, the generated
sources are mixing up function calls, so calls to specific GL functions
are being routed to the wrong call in glapi_mapi_tmp.h.