build: split CFLAGS_DEF and always add CFLAGS_MIN

Currently, when doing a custom "make CFLAGS=...", we run into a compile
error as several necessary defines are not set. Therefore, use override
to always set those.

Reported-by: Marco Belmonte <mbelmonte@globalmedia.com>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>