[build] fix more srcdir != builddir mistakes

I missed these in commit debae590e724ee064939982bd3f54a613d5fa650 for issue #2
because my test system had a compatible glxext.h in /usr/include/GL.  On a
system with an incompatible version (i.e. Ubuntu), this causes build failures.
Fix the include paths to include the GL headers from the repository instead of
the system.

Signed-off-by: Aaron Plattner <aplattner@nvidia.com>