CMake build under windows using NMake

[vasole]

Addresses #91

With these changes, the following instructions build SZ under windows with native MSVC compiler

mkdir build
cd build
cmake .. -G"NMake Makefiles"
nmake

[vasole]

I guess I can make first commit even more conservative (checking for windows without MINGW for instance).

Just let me now if you prefer so.

[disheng222]

Looks good to me. Thanks a lot for the effort.

[robertu94]

Thanks for the PR, it's good to have a Windows user contributing :) . A few things that need addressing:

[vasole]

Since the check for windows without using MINGW was already used at other places, I have used that approach to resolve the concerns about 73065ab

[robertu94]

This looks good to me now @disheng222