Off-tree builds

[Yuri D'Elia]

I was using a few hacks to build emacs off-tree, by configuring inside a
subfolder of the source to get a separate build tree.

This never worked quite right with bytecode and native compilation:
there are a few places where the location of the el/elc/eld files are
expected to be in the same tree. I was working this around with a few
symlinks so that I could still have the benefit of having different
builds for testing off a single source tree, but these hacks stopped
working lately.

I wonder if there's some general interest in possibly improving the
build system to support this, or nobody sees it as an advantage?