Add custom check for inet_pton()/inet_ntop() on MinGW/WIN32

Add custom check for inet_pton()/inet_ntop() on MinGW/WIN32

More recent MinGW versions have these functions (if compiling at
_VISTA level or higher), but the normal AC_CHECK_FUNCS() check does
not find them because the necessary header file is not #include'd and
the libws2_32 not linked - and our compat functions are incompatible
with the definitions in <ws2tcpip.h>, so compilation fails.

Fix with a custom AC_LINK_IFELSE()/AC_LANG_PROGRAM() construct.

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Tested-by: Heiko Hund <heiko.hund@sophos.com>
Tested-by: Samuli Seppänen <samuli@openvpn.net>
Lazy-ACK-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1442953884-54602-1-git-send-email-gert@greenie.muc.de>
URL: http://article.gmane.org/gmane.network.openvpn.devel/10165