[ci] Add build script for win

[frostming]

Related issue = #3391 #2715

1. Fix the cmake file to support in-venv build
2. Add a build script for windows as requested in Create a bash script for windows dev install  #3391 that:
   a. Usage: win_build.ps1 [[-libdir] <string>] [-clone] [-installVulkan] [-develop] [-install]
   b. Support rerun, won't download again if already exists
   c. build and install python library in a virtual environment to avoid contaminating the system site-packages
   d. support install in develop mode(with -develop flag), like python setup.py develop
   e. CMake args and environments are populated automatically, users don't bother to set them.
3. Replace the building steps in postsubmit and presubmit CI jobs with the script.

I've tested on Windows 10, CPU build, both build and install are successful. If you have a Windows PC, please help to test the script.

Other thoughts

Now setting up a taichi development environment on Windows is as easy as downloading a script from the internet and run it. We may also do the same for linux and mac. And development installation doc can be updated accordingly.

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[k-ye]

Note that the LLVM/clang/Vulkan libs usually are usually downloaded once and placed somewhere outside the repo dir/build dir, (i.e., a more stable place, e.g. C:\ThirdParty\Libs). I'd suggest that we add one additional flag to hold all these required dependent libs, WDYT?

In addition, let's make the default behavior not to download these libs?

[frostming]

the lib location can be specified with -build option, and if the library exists, won't download again.

So -build C:\ThirdParty\Libs can do the trick.

[k-ye]

I see, then let's give it a more obvious name, e.g. libsdir, depsdir, ...

[frostming]

The name is changed to libdir

[k-ye]

Huh, I guess the naming convention here is $libsDir? Also, the default value should be empty (or at least not _build)

[frostming]

I've updated the name and default to the current location (.) since empty isn't a valid path parameter

[k-ye]

Now setting up a taichi development environment on Windows is as easy as downloading a script from the internet and run it. We may also do the same for linux and mac. And development installation doc can be updated accordingly.

This would be a great step forward! We used to think about using conda to automate/simplify the process. Any suggestions on this :-) ?

[ailzhang]

Thanks!! I've tried this on a windows machine here and it worked like a charm!
Nit: is it possible to forward the real-time logs to the terminal when building taichi? ;) currently it stays silent for a while and then dumps all the logs at once :P