giflib: disable xmlto

Fixes:
 - http://autobuild.buildroot.org/results/87c7fa4a8e393acaccc84bd2774f9eee32ef7d90

xmlto is optionally used to generate documentation, which we don't need.

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/giflib/giflib.mk

@@ -18,6 +18,8 @@ GIFLIB_BINS = \
 	gifrsize gifspnge giftext giftool gifwedge icon2gif raw2gif rgb2gif \
 	text2gif
 
+GIFLIB_CONF_ENV = ac_cv_prog_have_xmlto=no
+
 define GIFLIB_BINS_CLEANUP
 	rm -f $(addprefix $(TARGET_DIR)/usr/bin/,$(GIFLIB_BINS))
 endef