[build] fix more srcdir != builddir mistakes
Created by: aaronp24
I missed these in commit debae590 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