Forcibly disable docs when building giflib (koreader#1186)

Because everything is terrible, I guess?

Fix koreader/koreader#6668

Author: NiLuJe

thirdparty/giflib/CMakeLists.txt

@@ -10,7 +10,7 @@ enable_language(C)
 ep_get_source_dir(SOURCE_DIR)
 ep_get_binary_dir(BINARY_DIR)
 
-set(CFG_ENV_VAR "CC=\"${CC}\" CFLAGS=\"${CFLAGS}\" LDFLAGS=\"${LDFLAGS}\" ")
+set(CFG_ENV_VAR "ac_cv_prog_have_xmlto=no CC=\"${CC}\" CFLAGS=\"${CFLAGS}\" LDFLAGS=\"${LDFLAGS}\"")
 set(CFG_OPTS "-q --prefix=${BINARY_DIR} --disable-static --enable-shared --host=\"${CHOST}\"")
 set(CFG_CMD sh -c "${CFG_ENV_VAR} ${SOURCE_DIR}/configure ${CFG_OPTS}")