xorg-server: needs mmu

os/utils.c::System() unconditionally uses fork(), so it only builds on
configurations with mmu.

(Indirectly) fixes:
http://autobuild.buildroot.net/results/497/4970352598a68d7af89acbb33cdf69d656e23f4d/
http://autobuild.buildroot.net/results/d27/d279dc35bff53b7f939af02bee70b1ed3014e49b/
http://autobuild.buildroot.net/results/84b/84b8f19221a5c841de3016959c99b92c96c57e81/

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/x11r7/xserver_xorg-server/Config.in

@@ -50,13 +50,14 @@ config BR2_PACKAGE_XSERVER_XORG_SERVER
 	select BR2_PACKAGE_XUTIL_UTIL_MACROS
 	select BR2_PACKAGE_XKEYBOARD_CONFIG
 	depends on !BR2_avr32
+	depends on BR2_USE_MMU # fork()
 	help
 	  X.Org X server
 
 	  http://xorg.freedesktop.org
 
 comment "xorg-xserver is BROKEN on AVR32"
-	depends on BR2_avr32
+	depends on BR2_avr32 && BR2_USE_MMU
 
 if BR2_PACKAGE_XSERVER_XORG_SERVER