An alternative solution to build on Windows

[tigercosmos]

As #18055 happens, I am still not able to build followed by README. Although @larsbergstrom tested it works as #18157, but I always test failed. I try to debug, but no progress.

I find an alternative solution to build on windows, and it works. I think it is worth to share, and let other developers who encountering problems while building have a new way.

Here's the steps:

1. Download and install Build Tools for Visual Studio 2017
2. Install python2.7 x86-x64 and virtualenv
3. Since mach.bat cannot find the prompt, we should execute Developer Command Prompt for VS 2017 manually in the Windows menu.
4. cd to/the/path/servo
5. python mach build -d

[tigercosmos]

If get error about rustc by unknown, #18145 can fix.

[shinglyu]

So the main difference you are proposing is step 3 to 5? If so (and if it's not possible to fix the vs shell anytime soon) I think it's worth adding a troubleshooting sub-section under the window build section in the README. Nice job finding an workaround.

[tigercosmos]

@shinglyu Step 1 is also different. Instead of installing VS 2017 IDE, I install VS 2017 tool only.

[larsbergstrom]

The work that @UK992 should have made it so that we work with the VS tools only and do not need Developer Command Prompt for VS 2017. @UK992 Do you have any opinions / ideas on this?

[tigercosmos]

But if I only install VS tool 2017, use cmd run mach.bat get

C:\Users\Tiger\Desktop\servo>mach.bat run
Visual Studio 2015 or 2017 is not installed.
Download and install Visual Studio 2015 or 2017 from https://www.visualstudio.com/

I guess mach.bat should be modified.