extra/tools: make sure autoconf can access pkg-config macros

If we don't build pkg-config (because a previous installation exists
somewhere else), autoconf won't find PKG_* m4 macros and fail to
run on packages using them (if they don't ship the macros either)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

extras/tools/tools.mak

@@ -132,7 +132,7 @@ autoconf: autoconf-$(AUTOCONF_VERSION).tar.bz2
 	$(UNPACK)
 	$(MOVE)
 
-.autoconf: autoconf
+.autoconf: autoconf .pkg-config
 	(cd $<; ./configure --prefix=$(PREFIX) && make && make install)
 	touch $@