#317 ZSTD compression

[David Beinder]

I have added ZSTD compression to NSIS, please see this PR for details on Github: https://github.com/kichik/nsis/pull/28

It would mean moving away from MSVC6, however I have also added changes that would restore Win9x compatibility on modern MSVC.

[Anders]

• SetPEWin95Supported should be called SetPEWinVerSupported since it applies to NT4 as well? It should pass in 4 for x86 stubs and plugins and 5 for AMD64.
• _allmul and _allshl should be hidden behind a ZSTD define so they are not used by accident in other places. We would also need the equivalent asm code for GCC?
• The Math plug-in can't statically link whatever it needs from the MS CRT?
• Ideally the MSVC support should be added in a separate step first.

(Githubs policy is blocking me from logging in on Github so I can't respond there)