build emacs outside of the source directory

[Stephen J. Turnbull]

Darren Hoo writes:

 > The built object files for emacs/{src,lib,lib-src} all go to
 > build/{src,lib,lib-src}, but the *.elc files are not, they still lie in
 > the emacs/lisp diretory.

It's complicated.  The theory is that compiled Lisp is universal (can
be run by any instance of Emacs) but that's not 100% true.
(Especially for *XEmacs because Mule is optional -- we just fixed a
related build bug, but that's not a problem here.)  To the extent that
the assumption is true, this is a significant build-time-saver.

The easy way to generalize this would be to symlink farm the lisp
directory into blddir instead of linking the directory itself.  But
for the reason above, and to some extent Andreas's point about tarball
distribution, it would need to be a configure option.  People might
object to that as needless complexity.