pkgconfig: Depend on opengl when built with libglvnd
I made a subtle mistake in the autoconf build, we actually want to depend on 'opengl' and not 'libglvnd' to get proper transitive library dependencies. (For context, https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=256997 which uses a basic Makefile build and links with libglu)
Before:
% pkg-config --libs glu
-L/usr/local/lib -lGLU
After:
% pkg-config --libs glu
-L/usr/local/lib -lGLU -lOpenGL