configure.ac: fix CFLAGS append
configure.ac: fix CFLAGS append

Some autotools/m4 versions seem to lack support of '+=' for appending
data to existing variables. This modification fixes this for the
final CFLAGS construction.

gebart/libcoap,authmillenon/libcoap,gebart/libcoap,gebart/libcoap,authmillenon/libcoap,authmillenon/libcoap