Consistently use the predefined _WIN32 and _WIN64 defines

The _WIN32 and _WIN64 defines are always defined automatically
by the compiler (both MSVC and mingw), so there's no need to
specify them in the project files.

The WIN32 defines might still be defined even if it isn't
defined manually, since some of the windows platform headers
defines it as well.
Add a proper include guard to dec_console.h
Remove "pragma once"

The same thing is handled by proper standard C header include guards
in all these files.
Remove commented out pragmas
Remove trailing whitespace in the mingw platform file