build(win): Fix windows.h pre-definitions

[lgritz]

• Avoid possible double define of NOMINMAX in platform.h
• Move other Windows symbol definitions into the cmake file instead of
  repeating them in certain .cpp files.

Fixes #3964

[jessey-git]

The changes to platform.h look good.

For the others inside the libutil project, you could consider doing the following in cmake, rather than pollute the source files with any of the #defines:
https://github.com/OpenImageIO/oiio/blob/master/src/libutil/CMakeLists.txt#L37
add_definitions(-DWIN32_LEAN_AND_MEAN -DNOMINMAX -DNOGDI -DVC_EXTRALEAN)

Up to you though. The current changes are also ok.

[lgritz]

Oh, good suggestion. Yes, I will do that.