Clarify vc11 detection in windows sdk
Win32 wants some definitions, otherwise it uses the latest versions of headers.

Upcoming TODO: letting _WIN32_WINNT default to 0x0602 (Win 8 SDK)
defines htonf, htond, etc. which doesn't end well.
Don't look for Perl on Windows

It's not used for terribly common things (test_rpc_gen), and
if you have miktex installed it's terribly broken.
Reduce duplication in finding windows sdk