Always define winversion

[obligaron]

MSVC doesn't define Winversion automatically, but asio want it to be defined.

This avoids the following build output (technically it's not a warning) from asio:

Please define _WIN32_WINNT or _WIN32_WINDOWS appropriately. For example:
- add -D_WIN32_WINNT=0x0601 to the compiler command line; or
- add _WIN32_WINNT=0x0601 to your project's Preprocessor Definitions.
Assuming _WIN32_WINNT=0x0601 (i.e. Windows 7 target).

[glebm]

We also do this in windows9x.cmake, hopefully this doesn't break it (don't see why it should)

[obligaron]

When both (windows and windows9x) would be added, it would break. Only the first definition stays (at least in MSVC; I tested it).

That's why I changed Platforms.cmake to only include "windows.cmake" or "windows9x.cmake".
As far as I have seen "windows9x.cmake" includes everything that is also in "windows.cmake" + some windows 9x specific changes.

[glebm]

Ah, makes sense, thanks!
These days treating windows 9x as a completely separate platform from modern Windows is sensible