dri: Require libudev-dev for building DRI on Linux.

The loader infrastructure for everything but DRI2 requires that udev be
present, so we can figure out an appropriate driver from the fd.  We don't
have a portable solution yet, but presumably it will have similar lookup
based on the device node.

It will also be even more required for krh's udev-based hwdb support,
which lets us have a loader that actually loads DRI drivers not included
in the loader's source distribution.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=75212
Reviewed-by: Matt Turner <mattst88@gmail.com>

configure.ac

@@ -994,6 +994,10 @@ if test "x$enable_dri" = xyes; then
             DEFINES="$DEFINES -DHAVE_DRI3"
         fi
 
+        if test "x$have_libudev" != xyes; then
+            AC_MSG_ERROR([libudev-dev required for building DRI])
+        fi
+
         case "$host_cpu" in
         powerpc* | sparc*)
             # Build only the drivers for cards that exist on PowerPC/sparc