configure.ac: Make --{without,with}-gallium-drivers work as expected
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 <2b237cafb16dc45038e85df6c85e74e6d899eba9@intel.com>
Reviewed-by: Dan Nicholson <2570c412b7cbb4203ff39baaca2bd49e72848226@gmail.com>

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