Use the standard _WIN32, not the Torism MS_WINDOWS or deprecated WIN32

This commit is completely mechanical; I used this perl script to make it:

 #!/usr/bin/perl -w -i.bak -p

 if (/^\s*\#/) {
     s/MS_WINDOWS/_WIN32/g;
     s/\bWIN32\b/_WIN32/g;
 }