We can't completely avoid it, since gl's headers use these defines for
their header guards, and we really do need to stop the system GL
headers from doing anything.
Fixes#14
Note that the generated code is still generated, they just aren't
built and installed. The goal with that is that someone could take
the built .c and .h files and drop it into their own project, if they
want to avoid shared libs.
This totally replaces the getprocaddress and dlsym code, which was
basically just stubs up until now. The is_glx/is_egl() is also
dropped -- they weren't doing anything, and the only false answer they
could give is if the dlopen were to fail.