# Only supported on Win32
unless ($^O eq "MSWin32" || $^O eq "cygwin") {
    die "OS unsupported\n"
}

# Must be Win32 version greater than 5
#   _WIN32_WINNT must be defined as greater than 5 for 
#   LockWorkStation prototype to be defined in winuser.h.
my $ver = `ver`;
$ver =~ / ([\d\.]+)/;
$ver = $1;

if ($ver < 5) {
    die "OS unsupported\n  Windows version must be greater than 5.\n  Found '$ver' with the 'ver' command."
}

WriteMakefile(
[...]