Incorporate Windows build instructions

[jimwins]

The Windows build instructions currently exist on the PHP Wiki but should be converted to DocBook format and replace the stub in install/windows/building.xml.

[cmb69]

Hmm, yeah, not really sure whether there should be manual build instructions (regardless of OS) in the PHP manual at all nowadays. I don't think that many users do those manual builds; and obviously there is quite some overlap with documentation for php-src and extension developers.

[jimwins]

Fair. Right now we have (some?) Unix build instructions in the manual, the Windows build instructions in the Wiki, and maybe more Unix build instructions in php-src (including something called docs-old/unix-build-system.md). Putting this in the php-src tree probably makes the most sense. So we could reverse this issue from 'move Windows build instructions to manual' to 'move Unix build instructions to php-src'.

[cmb69]

And we also have https://www.phpinternalsbook.com/php7/build_system.html, which had been written for PHP core and extension developers.

And to clarify: I think we should move the Windows build instructions off the Wiki. Actually, most of the Windows specific internal docs is grossly out-dated, so it doesn't make much sense to keep parts of it up to date.

My point about the manual Windows build instruction is, that we need to clarify whether these are for PHP core and extension developers, or for other people as well. If the latter, I would not recommend to install Visual Studio – the MS Build Tools will be completely sufficient. And when only building, quite something could be simplified. I don't know, though, if people are actually just rolling their own builds for Windows (one of the few reasons to do this would be to have PGO for an own application).