package/libdvdnav: fix build with gcc-4.8

Fixes:

  http://autobuild.buildroot.net/results/c9514368b98bc3a7f7ab92cb33c54bd3ffd9a232/

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/libdvdnav/libdvdnav.mk

@@ -8,6 +8,7 @@ LIBDVDNAV_VERSION = 6.1.0
 LIBDVDNAV_SOURCE = libdvdnav-$(LIBDVDNAV_VERSION).tar.bz2
 LIBDVDNAV_SITE = http://www.videolan.org/pub/videolan/libdvdnav/$(LIBDVDNAV_VERSION)
 LIBDVDNAV_INSTALL_STAGING = YES
+LIBDVDNAV_CONF_ENV = CFLAGS="$(TARGET_CFLAGS) -std=gnu99"
 LIBDVDNAV_DEPENDENCIES = libdvdread host-pkgconf
 LIBDVDNAV_LICENSE = GPL-2.0+
 LIBDVDNAV_LICENSE_FILES = COPYING