Avoid excessive warnings about redefined macros

Avoid excessive warnings about redefined macros

I can't vouch for the correctness of the patch. However, it does succeed in eliminating the endless stream of warnings of the form:

    gcc -DHAVE_CONFIG_H -I. -I../../..  -I../../../smtpd -I../../../openbsd-compat -I../../../contrib/lib/libc/asr -D_FORTIFY_SOURCE=2 -DASR_OPT_THREADSAFE=0 -DNO_IO -DBUILD_FILTER -c -o ../../../smtpd/filter_dnsbl-util.o `test -f '../../../smtpd/util.c' || echo './'`../../../smtpd/util.c
    In file included from ../../../openbsd-compat/includes.h:75:0,
                     from ../../../smtpd/util.c:22:
    ../../../openbsd-compat/defines.h:377:23: warning: extra tokens at end of #undef directive [enabled by default]
     # undef _PATH_MAILDIR MAILDIR
                           ^

Please consider applying it or a variant thereof.

Patch by Benny Baumann <BenBE@geshi.org>. Originally submitted as Debian Bug 747666: https://bugs.debian.org/747666