glx: drop GLX_LIBS from X server and workaround sdksyms.

We've had reports of two copies of the GLX bits, one in the server
and one in libglx.so causing problems, I didn't understand why the
X server needed a copy so drop it, however then we have to fix a missing
GlxExtensionInit that comes from sdksyms, so work around it by moving
that one declaration into a header that sdksyms doesn't scan.

Thanks to Jon Turney for debugging the actual problem.
(copyright header from extinit.h that seems most appropriate put on top).

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=52402
Tested-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Daniel Stone <daniel@fooishbar.org>
Tested-by: Jon TURNEY <jon.turney@dronecode.org.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Keith Packard <keithp@keithp.com>

include/extinit.h

@@ -83,7 +83,6 @@ extern void GEExtensionInit(void);
 
 #ifdef GLXEXT
 extern _X_EXPORT Bool noGlxExtension;
-extern void GlxExtensionInit(void);
 #endif
 
 #ifdef PANORAMIX