configure: enable dri3 only for linux

Currently only linux can make use of dri3, so it would make sense to
enable it explicitly for the platform.
Drop a duplicated libudev check while we're at it.

v3: Properly handle dri3 and reword commit message.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=76377
Cc: "10.1" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>

configure.ac

@@ -527,11 +527,20 @@ AC_ARG_ENABLE([dri],
         [enable DRI modules @<:@default=enabled@:>@])],
     [enable_dri="$enableval"],
     [enable_dri=yes])
+
+case "$host_os" in
+linux*)
+    dri3_default=yes
+    ;;
+*)
+    dri3_default=no
+    ;;
+esac
 AC_ARG_ENABLE([dri3],
     [AS_HELP_STRING([--enable-dri3],
-        [enable DRI3 @<:@default=enabled@:>@])],
+        [enable DRI3 @<:@default=auto@:>@])],
     [enable_dri3="$enableval"],
-    [enable_dri3=yes])
+    [enable_dri3="$dri3_default"])
 AC_ARG_ENABLE([glx],
     [AS_HELP_STRING([--enable-glx],
         [enable GLX library @<:@default=enabled@:>@])],
@@ -823,9 +832,6 @@ xyesno)
         PKG_CHECK_MODULES([DRI2PROTO], [dri2proto >= $DRI2PROTO_REQUIRED])
         GL_PC_REQ_PRIV="$GL_PC_REQ_PRIV libdrm >= $LIBDRM_REQUIRED"
         if test x"$enable_dri3" = xyes; then
-            if test x"$have_libudev" != xyes; then
-              AC_MSG_ERROR([DRI3 requires libudev >= $LIBUDEV_REQUIRED])
-            fi
             PKG_CHECK_MODULES([DRI3PROTO], [dri3proto >= $DRI3PROTO_REQUIRED])
             PKG_CHECK_MODULES([PRESENTPROTO], [presentproto >= $PRESENTPROTO_REQUIRED])
         fi