Add building/testing with msbuild and the clang compiler

Add building/testing with msbuild and the clang compiler

The LLVM/clang compiler warning and error message are easier too read
than their MSVC cl counterparts. Also compiling/running tests on Windows
with a different compiler has the benefit of a better coverage.

This includes a few minor changes to allow clang-cl to compile the
project.

Change-Id: I43d84034f3e920a45731c4aab4f851a60921290d
Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Lev Stipakov <lstipakov@gmail.com>
Message-Id: <20241227112209.11572-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg30231.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>