Restore OpenGL programmable pipeline support.

Programmable pipeline support with OpenGL (not OpenGLES) was broken in
commit 691ba74d0febbeb2350aacab243557257e376b31.

All the programmable pipeline support was hidden behind

    #ifndef ALLEGRO_CFG_NO_GLES2

which when trivially inverted became

    #ifdef ALLEGRO_CFG_OPENGLES2

but ALLEGRO_CFG_NO_GLES2 was also NOT defined for OpenGL proper.
Revert mistake in previous commit d0395e5.