Check for an OpenGL implementation. If GL is found, the required compiler and linker flags are included in the output variables GL_CFLAGS and GL_LIBS, respectively. If no usable GL implementation is found, no_gl is set to yes.

If the header GL/gl.h is found, HAVE_GL_GL_H is defined. If the header OpenGL/gl.h is found, HAVE_OPENGL_GL_H is defined. These preprocessor definitions may not be mutually exclusive.