pipe-loader: wrap pipe_loader_sw_probe_xlib within HAVE_PIPE_LOADER_XLIB

pipe-loader: wrap pipe_loader_sw_probe_xlib within HAVE_PIPE_LOADER_XLIB

The above function implies using the the xlib winsys, which
has additional library dependencies that should not be forced.

Make the software xlib pipe loader optional thus avoid all
the dependency hell. A user that wishes to use the particular
pipe-loader would need to set the following within configure.ac.

 enable_gallium_xlib_loader=yes

v2:
 - Wrap sw/xlib/xlib_sw_winsys.h to handle compilation on systems
lacking X11 headers. Spotted by Christian Prochaska.

Tested-by: Tom Stellard <thomas.stellard@amd.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=75356
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>