glib-networking: fix build without gnutls

configure errors out when it doesn't find gnutls unless explicitly
disabled.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/glib-networking/glib-networking.mk

@@ -16,7 +16,9 @@ GLIB_NETWORKING_DEPENDENCIES = $(if $(BR2_NEEDS_GETTEXT_IF_LOCALE),gettext libin
 
 ifeq ($(BR2_PACKAGE_GNUTLS),y)
 GLIB_NETWORKING_DEPENDENCIES += gnutls
-GLIB_NETWORKING_CONF_OPT = --with-libgcrypt-prefix=$(STAGING_DIR)/usr
+GLIB_NETWORKING_CONF_OPT += --with-libgcrypt-prefix=$(STAGING_DIR)/usr
+else
+GLIB_NETWORKING_CONF_OPT += --without-gnutls
 endif
 
 $(eval $(call AUTOTARGETS))