package/dhcp: fix build with gcc-15.x

Signed-off-by: Bernd Kuhls <bernd@kuhls.net>
Signed-off-by: Julien Olivain <ju.o@free.fr>

package/dhcp/dhcp.mk

@@ -27,10 +27,17 @@ define DHCP_LIBTOOL_AUTORECONF
 	cp $(@D)/configure.ac+lt $(@D)/configure.ac
 endef
 
+# gcc-15 defaults to -std=gnu23 which introduces build failures.
+# We force "-std=gnu17" for gcc version supporting it. Earlier gcc
+# versions will work, since they are using the older standard.
+ifeq ($(BR2_TOOLCHAIN_GCC_AT_LEAST_8),y)
+DHCP_GCC_OPTS = -std=gnu17
+endif
+
 DHCP_CONF_ENV = \
 	CPPFLAGS='-D_PATH_DHCPD_CONF=\"/etc/dhcp/dhcpd.conf\" \
 		-D_PATH_DHCLIENT_CONF=\"/etc/dhcp/dhclient.conf\"' \
-	CFLAGS='$(TARGET_CFLAGS) -DISC_CHECK_NONE=1'
+	CFLAGS='$(TARGET_CFLAGS) -DISC_CHECK_NONE=1 $(DHCP_GCC_OPTS)'
 
 DHCP_BIND_EXTRA_CONFIG = \
 	--build=$(GNU_HOST_NAME) \