Use _WIN32 instead of WIN32 preprocessor macro

Use _WIN32 instead of WIN32 preprocessor macro

- _WIN32 is the recommended preprocessor macro
- Substitute occurrences of WIN32 with _WIN32 using:
git grep -lw -e '#ifdef WIN32' -e '#ifndef WIN32' -e 'defined(WIN32)' \
-- '*.c' '*.h' '*.xs' | xargs \
sed -i -e 's/#ifdef WIN32/#ifdef _WIN32/g' \
-e 's/#ifndef WIN32/#ifndef _WIN32/g' \
-e 's/defined(WIN32)/defined(_WIN32)/g'