Revert "dhcpcd: fix ipv6 related compile failure"

This reverts commit 6b1f66643d782fc6f00e7436d41ceea6d88ad6bf.

It turns out to not really fix the build errors after all, so revert it.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/dhcpcd/dhcpcd.mk

@@ -20,8 +20,8 @@ endif
 
 define DHCPCD_CONFIGURE_CMDS
 	(cd $(@D); \
-	$(TARGET_CONFIGURE_OPTS) CFLAGS="$(TARGET_CFLAGS) -D_GNU_SOURCE" \
-		./configure --os=linux \
+	$(TARGET_CONFIGURE_OPTS) ./configure \
+		--os=linux \
 		$(DHCPCD_CONFIG_OPTS) )
 endef