configure: error out when building xa only with swrast

Building to provide accelration using swrast does not make
sense.

Note: update your build script to explicitly mention svga
in the gallium drivers list, if you are building the vmwgfx
xa library.

v2: Update error message to provide more clarify, add an example.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
1 file changed