Also already mentioned in ba4338d6f7,
the security feature FORTIFY_SOURCE could be set globally.
The current approach tries to undefine and redefine it, however,
the order on the command line prevents this from being successfully, e.g.:

arm-openwrt-linux-muslgnueabi-gcc  ... -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2 ... -D_FORTIFY_SOURCE=1 ...
                                       `---------------v-------------------'     `--------v--------'
                                                   AM_CFLAGS                        OpenWrt params

So to have the original source (static) _FORTIFY_SOURCE=2 define,
the only option is to remove it from AM_CFLAGS completely, and
to pass it after the OpenWrt settings explicitely in the Makefile.

This should fix the buildbots finally.

Signed-off-by: Michael Heimpold <mhei@heimpold.de>