bug#20968: 25.0.50; Be able to specify the output directory for `byte-compile-file'

[Drew Adams]

Enhancement request.

1. Be able to specify the output directory for *.elc files to
`byte-compile-file'.  See this emacs.SE question:
http://emacs.stackexchange.com/q/13596/105.

This could be done by adding an optional arg, but to let users take
advantage of it interactively the current prefix-arg behavior would need
to be split.  E.g., numeric value <= 0 could mean prompt for the output
dir; numeric value >= 0 could mean also load the byte-compiled file.

That would not hurt any existing code that passed a non-nil and
non-numeric second arg.

Another possibility would be to define another command for this, e.g.,
`byte-compile-file-to-directory'.

2. Let commands such as `dired-do-byte-compile' (hence function
`dired-byte-compile') support this also, e.g. by a particular prefix-arg
value that does not conflict (much) with the current prefix-arg
behavior.  For example, plain `C-u' - a user can still use `M-4' to get
the current `C-u' behavior.

3. In addition, maybe provide a way (e.g. option) for users to specify a
default value for the directory.  #1 and #2 would provide this as the
default when you are asked to specify the output dir.
        

In GNU Emacs 25.0.50.1 (i686-pc-mingw32)
 of 2015-05-29 on LEG570
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --prefix=/mingw32 --host=i686-pc-mingw32
 --enable-checking=yes,glyphs'