glapi: Build glapi_gentable.c only on Darwin

glapi: Build glapi_gentable.c only on Darwin

Removes the public symbol _glapi_create_table_from_handle from
libGL.so.1.2.0 on all platforms except Darwin.

Since the symbol is not used on other platforms it makes sense to
build glapi_gentable.c only on Darwin.

As a side effect it accelerates the build a bit and reduces the size
of libGL.so.1.2.0 as follows:

size lib/libGL.so.1.2.0 on my system shows
   text	   data	    bss	    dec	    hex	filename
 469211	  21848	   2720	 493779	  788d3	lib/libGL.so.1.2.0 before
 420988	  11240	   2720	 434948	  6a304	lib/libGL.so.1.2.0 after

A little bit of history:

_glapi_create_table_from_handle was introduced in

commit 85937f4c0d4a78d3a11e3c1fa6148640f2a9ad7b
Author: Jeremy Huddleston <jeremyhu@apple.com>
Date:   Thu Jun 9 16:59:49 2011 -0700

    glapi: Add API that can create a _glapi_table from a dlfcn handle

    Example usage:

    void *handle = dlopen(opengl_library_path, RTLD_LOCAL);
    struct _glapi_table *disp = _glapi_create_table_from_handle(handle,
"gl");

    Signed-off-by: Jeremy Huddleston <jeremyhu@apple.com>
and the only user in mesa was added in

commit f35913b96e743c5014e99220b1a1c5532a894d69
Author: Jeremy Huddleston <jeremyhu@apple.com>
Date:   Thu Jun 9 17:29:51 2011 -0700

    apple: Use _glapi_create_table_from_handle to initialize our
dispatch table

    Signed-off-by: Jeremy Huddleston <jeremyhu@apple.com>
gl_gentable.py was also used for XQuartz in xserver 1.11 - 1.14.

v2: Fix typos in commit message
    Add missing XORG_GLAPI_OUTPUTS += \ into src/mapi/glapi/gen/Makefile.am
    Add glapi_gentable.c to EXTRA_DIST for inclusion in the release
    tarball

v3: Fix commit message: s/gl_gentable.c/glapi_gentable.c/

Reported-by: Arlie Davis <arlied@google.com>
Cc: Jeremy Huddleston <jeremyhu@apple.com>
Signed-off-by: Andreas Boll <andreas.boll.dev@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>