Use the logging framework instead of print, to be consistent and as the logging framework prints out messages unbuffered which is more responsive on Windows.

Add debug logging facility to track waits on external processes when EM_BUILD_VERBOSE >= 3. This helps pinpointing if the build hangs on some tool dying/live/deadlocking, and where it might occur.

Implement process.pid on WindowsPopen replacement so that EM_BUILD_VERBOSE=3 works on it as well.
Don't do Popen workaround on Windows by default, since the cmdline -> call emcc.bat usage doesn't seem to need it, and it adversely affects the logging buffering that makes the compiler look unresponsive, since it will only print out compilation output messages at the very end of the whole run.
Merge pull request #1631 from juj/responsive_logging

Responsive logging