package/libtasn1: fix build with gcc 4.8

Fixes:
 - http://autobuild.buildroot.org/results/0030c903abf6c964806a97067af94a99867a3896

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/libtasn1/libtasn1.mk

@@ -17,4 +17,6 @@ LIBTASN1_AUTORECONF = YES
 # 'missing' fallback logic botched so disable it completely
 LIBTASN1_CONF_ENV = MAKEINFO="true"
 
+LIBTASN1_CONF_OPTS = CFLAGS="$(TARGET_CFLAGS) -std=gnu99"
+
 $(eval $(autotools-package))