Source: Fix typo in _WIN32 preprocessor checks

Since CMake's default compiler flags with MSVC include -DWIN32 for historical reasons, a few preprocessor conditions were accidentally checking for WIN32 instead of _WIN32. The corresponding blocks were left out when compiling official binaries for cmake.org because we hard-code compiler flags without -DWIN32.

Fixes: #22764 (closed)
Backport: release