autoconf: Improve the visibility of the swrast DRI driver

Improve the --with-dri-drivers help text so that users are aware that
they should install the swrast DRI driver.

configure.ac

@@ -604,7 +604,8 @@ fi
 dnl Which drivers to build - default is chosen by platform
 AC_ARG_WITH([dri-drivers],
     [AS_HELP_STRING([--with-dri-drivers@<:@=DIRS...@:>@],
-        [comma delimited DRI drivers, e.g. "i965,radeon,nouveau" @<:@default=auto@:>@])],
+        [comma delimited DRI drivers list, e.g.
+        "swrast,i965,radeon,nouveau" @<:@default=auto@:>@])],
     [with_dri_drivers="$withval"],
     [with_dri_drivers=yes])
 if test "x$with_dri_drivers" = x; then

docs/autoconf.html

@@ -182,11 +182,12 @@ will search for DRI drivers. The default is <code>${libdir}/dri</code>.
 </li>
 <li><code>--with-dri-drivers=DRIVER,DRIVER,...</code> - This option
 allows a specific set of DRI drivers to be built. For example,
-<code>--with-dri-drivers="i965,radeon,nouveau"</code>. By default,
-the drivers will be chosen depending on the target platform. See the
-directory <code>src/mesa/drivers/dri</code> in the source tree for
-available drivers.
-</li>
+<code>--with-dri-drivers="swrast,i965,radeon,nouveau"</code>. By
+default, the drivers will be chosen depending on the target platform.
+See the directory <code>src/mesa/drivers/dri</code> in the source tree
+for available drivers. Beware that the swrast DRI driver is used by both
+libGL and the X.Org xserver GLX module to do software rendering, so you
+may run into problems if it is not available.</li>
 <!-- This explanation might be totally bogus. Kristian? -->
 <li><code>--disable-driglx-direct</code> - Disable direct rendering in
 GLX. Normally, direct hardware rendering through the DRI drivers and