When fortify source is enabled, the c-ares configure script will abort with:
    configure: CFLAGS error: CFLAGS may only be used to specify C compiler flags, not macro definitions. Use CPPFLAGS for: -D_FORTIFY_SOURCE=1

Change the OpenWrt Makefile to move any -D flags from TARGET_CFLAGS to
TARGET_CPPFLAGS in order to satisfy `configure`.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>