#14 Consistently use '_WIN32' macro, never 'WIN32'

[Max Barraclough]

This patch replaces every reference to the 'WIN32' macro with a reference to the '_WIN32' macro. Previously, both WIN32 and _WIN32 were used. _WIN32 is always defined by MSVC itself, unlike WIN32.

This change obviates the need for the Windows build configuration to define the WIN32 macro.

See also:
https://learn.microsoft.com/en-us/cpp/preprocessor/predefined-macros

[Max Barraclough]

This patch is for FOX 1.7.81.