Contribs: use predefined variable for VideoLAN server

(cherry picked from commit eca4d24a3457a34613d55da2ec4520156dd330c4)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

contrib/src/dvdnav/rules.mak

 # DVDNAV
 
 LIBDVDNAV_VERSION := 5.0.0
-LIBDVDNAV_URL := http://download.videolan.org/pub/videolan/libdvdnav/$(LIBDVDNAV_VERSION)/libdvdnav-$(LIBDVDNAV_VERSION).tar.bz2
+LIBDVDNAV_URL := $(VIDEOLAN)/libdvdnav/$(LIBDVDNAV_VERSION)/libdvdnav-$(LIBDVDNAV_VERSION).tar.bz2
 
 ifdef BUILD_DISCS
 ifdef GPL

contrib/src/dvdread/rules.mak

 # DVDREAD
 LIBDVDREAD_VERSION := 5.0.0
-LIBDVDREAD_URL := http://download.videolan.org/pub/videolan/libdvdread/$(LIBDVDREAD_VERSION)/libdvdread-$(LIBDVDREAD_VERSION).tar.bz2
+LIBDVDREAD_URL := $(VIDEOLAN)/libdvdread/$(LIBDVDREAD_VERSION)/libdvdread-$(LIBDVDREAD_VERSION).tar.bz2
 
 ifdef BUILD_DISCS
 ifdef GPL