_MSC_VER -> _WIN32

[DennisHeimbigner]

It appears that at some point, cygwin's gcc changed to no longer define _WIN32.
So we should begin experimenting with converting all references to _MSC_VER
with _WIN32 (or in some cases _WIN64).
It would be nice to know what changed and when so that we can
do some ./configure tests for it.
Also need to see if this works under cmake using gcc.

re: #1105

[WardF]

@DennisHeimbigner , I'm going through and cleaning up issues that were associated with 4.6.2. Is this still an issue?

[DennisHeimbigner]

I think so. I have been occasionally converting _MSC_VER to _WIN32 as
part of various pull requests. But there are still a significant number of occurrences.