GUB / autoconf out-of-tree builds

[Graham Percival]

I've tracked down the srcdir vs. builddir problem to autoconf.

builddir = 
~/gub/target/linux-x86/build/lilypond-git.sv.gnu.org--lilypond.git-master$ 

srcdir = 
~/gub/target/linux-x86/src/lilypond-git.sv.gnu.org--lilypond.git-master$ 


builddir contains a GNUmakefile, which consists of:
----
depth = ./
include config$(if $(conf),-$(conf),).make
include $(configure-srcdir)/GNUmakefile.in
----

it obviously requires the srcdir GNUmakefile.in, but running:
  make DOCUMENTATION=YES dist
fails, because the GNUmakefile in the srcdir contains:
----
dist-toplevel-txt-files: top-doc
        -mkdir -p $(distdir)
        ln $(TOPDOC_TXT_FILES) $(distdir)/
        ln $(top-src-dir)/stepmake/aclocal.m4 $(distdir)/
----

The $(TOPDOC_TXT_FILES) includes RELEASE-COMMIT, which is only
created in the builddir, not the srcdir.


I've never used autoconf to do an out-of-tree build before, so I'm
getting lost.  I've done this in cmake, but it's trivial to do
with cmake.  :|

I'd like to make a 2.13.3 release soon, so any advice would be
appreciated.
(actually, I suppose the easiest thing would be to remove the
RELEASE-COMMIT stuff from the normal lilypond git)

Cheers,
- Graham