gcc: enable poison system directories option

This commit enables the poison system directories option, which is now
available thanks to the gcc patches that have been added.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: Romain Naour <romain.naour@openwide.fr>
Tested-by: Romain Naour <romain.naour@openwide.fr>

package/gcc/gcc-final/gcc-final.mk

@@ -50,6 +50,7 @@ HOST_GCC_FINAL_CONF_OPTS = \
 	$(HOST_GCC_COMMON_CONF_OPTS) \
 	--enable-languages=$(GCC_FINAL_CROSS_LANGUAGES) \
 	$(DISABLE_LARGEFILE) \
+	--enable-poison-system-directories \
 	--with-build-time-tools=$(HOST_DIR)/usr/$(GNU_TARGET_NAME)/bin
 
 # Disable shared libs like libstdc++ if we do static since it confuses linking