Fix Building Using MSVC

Fix Building Using MSVC

This patch enables the building of OpenVPN for the 2.4 and master
branches using MSVC (Visual Studio 2013 / MSVC v120), which currently
doesn't work with 2.4 or a clone of master. 2013 is being used as it
reduces the complexity of the redistributable requirements and has
mostly complete C99 support. Further changes will be necessary for 2015
support when the switch is made.

Note the changes to config-msvc-version.h.in are more of a work around.
It was a simpler approach when compared to modifying msvc-generate.js to
handle m4 syntax, and so it may be dropped if there is an intention to
update the javascript generator.

Signed-off by: Eric Thorpe <eric@sparklabs.com>

Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <df273b9b-6ca4-a539-cdf5-d4f9f991896b@sparklabs.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg14268.html

Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 5ab106db7b091c6409fd0a7e43f557a7931c200f)