Kernel modesettings support also depends on dri2, see
http://cgit.freedesktop.org/xorg/xserver/tree/hw/xfree86/drivers/modesetting/Makefile.am#n46

Patch sent upstream: https://gitlab.freedesktop.org/xorg/xserver/issues/479

[rebased for version 1.20.0]
Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>

diff -uNr xorg-server-1.17.2.org/configure.ac xorg-server-1.17.2/configure.ac
--- xorg-server-1.17.2.org/configure.ac	2015-06-16 17:42:40.000000000 +0200
+++ xorg-server-1.17.2/configure.ac	2015-08-08 10:44:59.702382624 +0200
@@ -1962,7 +1962,7 @@
 	        XORG_SYS_LIBS="$XORG_SYS_LIBS $XORG_MODULES_LIBS"
 	fi
 
-	if test "x$DRM" = xyes; then
+	if test "x$DRM" = xyes -a "x$DRI2" = xyes; then
 		XORG_DRIVER_MODESETTING=yes
 	fi