targets/xa: provide alternative(static) xa target

targets/xa: provide alternative(static) xa target

Now we can build the xa target (libxatracker) with either static
pipe-drivers or shared ones. Currently we default to static.

 - Remove the unused CFLAGS/CPPFLAGS.
 - Use GALLIUM_TARGET_CFLAGS where applicable.

v2: Update the printout messages at configure.
v3: Drop inclusion of the wrapper winsys and softpipe/llvmpipe.

Cc: Jakob Bornecrantz <jakob@vmware.com>
Cc: Rob Clark <robclark@freedesktop.org>
Cc: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>