[CM] compiling Snd --with-gl version

[Bill Schottstaedt]

> gl.c:5030: `GL_TEXTURE_BINDING_3D' undeclared (first use in this

These look like GL version problems.  The first thing I'd try
is --with-just-gl (to leave out gl.c) -- the code used by the
spectrogram is actually very basic stuff and ought to work
in any version (though I haven't tried it).  I'll look into
making gl.c compatible with older versions of GL.