Add MIN() compatibility macro

commit 3b23b18dddb8f8f4a introduced usage of MIN(), and not all platforms
have the macro in "readily usable" system header files, most notably it's
missing on OpenSolaris and Android.  Add macro if not defined yet.

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20130323112234.GM17727@greenie.muc.de>
URL: http://article.gmane.org/gmane.network.openvpn.devel/7450