package/x11r7/xserver_xorg-server: libunwind support is broken on mips64

Fixes
http://autobuild.buildroot.net/results/e8f/e8f68163802f31359cee15fa3ca52547e06d1f27/
http://autobuild.buildroot.net/results/994/9947222da8682a358f81f8ac924c12596fd09ee8/
http://autobuild.buildroot.net/results/df9/df92d9b682976b77d9e1e4c39afca52b7512d847/

[Peter: use official bugtracker URL, fix check]
Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/x11r7/xserver_xorg-server/xserver_xorg-server.mk

@@ -144,7 +144,9 @@ ifeq ($(BR2_PACKAGE_FREETYPE),y)
 XSERVER_XORG_SERVER_DEPENDENCIES += freetype
 endif
 
-ifeq ($(BR2_PACKAGE_LIBUNWIND),y)
+# libunwind support is broken on mips64
+# https://bugs.freedesktop.org/show_bug.cgi?id=79939
+ifeq ($(BR2_PACKAGE_LIBUNWIND)-$(BR2_mips64el),y-)
 XSERVER_XORG_SERVER_DEPENDENCIES += libunwind
 else
 XSERVER_XORG_SERVER_CONF_OPTS += --disable-libunwind