Allow running ./configure and Makefile on Windows BASH

[devappd]

Support is built in emsdk-npm@a019e59 and I'm tracking branch windows-bash-make.

For now, I'm stalling this unless there's interest.

Issues on AppVeyor:

cd C:\emsdk
. C:\emsdk\emsdk_env.sh

• Running C:\Program Files\Git\usr\bin\bash.exe on these commands causes AppVeyor's Python 3.8 to go to interactive shell. emsdk_env.sh is passing all arguments correctly. Somehow Python is not receiving them. AppVeyor's Python 2.7 DOES behave as expected, but I want to avoid switching PATHs. emsdk_env.sh --> ./emsdk searches for python3 first.

Other:

• I did not make this configurable yet -- i.e., windowsBash: true or windowsBash: path/to/shell.exe