bug#41242: Port feature/native-comp to Windows

[Eli Zaretskii]

> From: Nicolas Bértolo <nicolasbertolo@gmail.com>
> Date: Tue, 19 May 2020 16:25:53 -0300
> Cc: Andrea Corallo <akrl@sdf.org>, 41242@debbugs.gnu.org
> 
> * lisp/emacs-lisp/comp.el (comp-effective-async-max-jobs): Use
> NUMBER_OF_PROCESSORS environment variable if system is Windows NT,
> "nproc" if it is in PATH or a default of 1.

This shouldn't be necessary: we already have a function to determine
the number of processors, see get_native_system_info in w32.c.  If you
need the result exported to Lisp, we can define a new variable which
will be populated with the value.

> Subject: [PATCH] Determine the emacs root dir only when necessary.
> 
> * src/fileio.c: Introduce function emacs_root_dir(). Refactor
> `expand-file-name` to use it.
> * src/lisp.h: Separate emacs_root_dir() into dos_emacs_root_dir() and
> w32_emacs_root_dir().
> * src/msdos.c: Rename emacs_root_dir() to dos_emacs_root_dir().
> * src/w32.c: Rename emacs_root_dir() to w32_emacs_root_dir().

Can you explain what problem this solves, and how?  It is especially
important to understand when will be emacs_root_dir first called
during a session.  That's because it calls filename_from_ansi, which
AFAIR needs some setup that happens at the beginning of a session.

Thanks.