Update vswhere to 3.1.7

[cmb69]

I've fetched the exe from https://github.com/microsoft/vswhere/releases/tag/3.1.7, and tried the vc15, vc16 and vs17 startup scripts without any noticable issues.

[cmb69]

Ah, forgot: as of vswhere 3.0.1, Windows 7 is required. That's not an issue for PHP 7.2.0 and above, but older PHP versions couldn't be properly built anymore (at least not out-of-the-box). If we drop that support, we probably could also drop support for vc14, and maybe remove other stuff as well.

[cmb69]

If we drop that support, we probably could also drop support for vc14, and maybe remove other stuff as well.

Well, that wouldn't buy us much; while we could get rid of the registry lookups in phpsdk_setshell.bat, that code isn't a particular issue, but we would remove the possibility to build somewhat newer PHP versions with vc14, what might work (or not, haven't tested). Furthermore Visual Studio 2015 has still extended support until Okt 2025, so we may reconsider in a year or two.

Anyhow, tested with vswhere 3.1.7 for a week, and haven't noticed any issues, so I'm going to merge this PR.