libgl-xlib: softpipe and llvmpipe aren't mutually exclusive at link time

libgl-xlib: softpipe and llvmpipe aren't mutually exclusive at link time

Since automake changes, softpipe and llvmpipe are mutually exclusive at link
time.  This doesn't make much sense to me as we can choose between them at
run-time using GALLIUM_DRIVER.

Creating library file: .libs/libGL.dll.a
.libs/xlib.o: In function `sw_screen_create_named':
/jhbuild/checkout/mesa/mesa/src/gallium/targets/libgl-xlib/../../../../src/gallium/auxiliary/target-helpers/inline_sw_helper.h:35:
undefined reference to `_softpipe_create_screen'

Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>
Reviewed-by: Brian Paul <brianp@vmware.com>