configure: disable shared glapi when building xlib powered glx

configure: disable shared glapi when building xlib powered glx

With commit 0432aa064b(configure: use shared-glapi when more than one
gl* API is used) we removed "disable shared-glapi when building without
dri" hunk.

In the good old days of classic mesa, dri and xlib-glx were mutually
exclusive thus the hunk made sense.

Currently enable-dri is used as a synonym for a range of things thus
it's more appropriate to handle xlib-glx explicitly.

Fixes a missing symbol '_glapi_Dispatch' in a xlib powered libGL,
build using the following

 ./autogen.sh --enable-xlib-glx --disable-dri --with-gallium-drivers=swrast

Cc: Brian Paul <brianp@vmware.com>
Reported-by: Brian Paul <brianp@vmware.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>