configure.ac: Make --{without,with}-gallium-drivers work as expected

This version is mostly Dan's post to the mesa-dev mailing list on
6/22/2011.

NOTE: This is a candidate for the 7.10 and 7.11 branches.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Dan Nicholson <dbn.lists@gmail.com>
1 file changed