fake:

package/x11/mozilla/mozilla.conf

@@ -64,10 +64,12 @@ export MOZILLA_OFFICIAL=1
 mk_add_options BUILD_OFFICIAL=1
 mk_add_options MOZILLA_OFFICIAL=1
 EOT
-for lib in zlib libjpeg libpng libmng ; do
-	pkginstalled $lib && echo ac_add_options --with-system-${lib#lib} >> .mozconfig
-done
 
+	for lib in zlib libjpeg libpng libmng ; do
+		if pkginstalled $lib ] ; then
+			echo ac_add_options --with-system-${lib#lib} >> .mozconfig
+		fi
+	done
 }
 
 moz_build() {