autoconf: Configurable DRI drivers

autoconf: Configurable DRI drivers

The user can request specific DRI drivers to build rather than the
default of "all that build on this platform". This allows the list of
drivers to be easily slimmed down.

This is controlled through the option --with-dri-drivers. For example:

    ./configure --with-driver=dri --with-dri-drivers="i965,nouveau"

Unfortunately, using this setting means the DRI drivers aren't filtered
by platform anymore and you might try to build something that doesn't
work.