AC_OUTPUT eats VPATH

[Rasmus Tamstorf]

Hi,

AC_OUTPUT seems a little agressive in terms of optimizing VPATH away from
makefiles. Looking at the code in acgeneral.m4 :

  # VPATH is dangerous, but if there is a colon in the path, we need to
  # keep it.
  if test "x$srcdir" = x.; then
    ac_vpsub=['/^[        ]*VPATH[        ]*=[^:]*$/d']
  fi

it looks like the VPATH statement gets taken out whenever @srcdir@ is '.'
unless there is a colon in the path. 

However, I'd like to do 

  VPATH = ../@srcdir@

which gets eaten unless I change it to 

  VPATH = ../@srcdir@:.

Now, why do I want to do this ? Well, I'm trying to set up a
multi-architecture build system similar to what's described on 

  http://www.paulandlesley.org/gmake/multi-arch.html

such that the build for each architecture happens in a subdirectory of the
source directory. Using something like : 

  AC_OUTPUT([src/_$ARCH/Makefile:src/Makefile.in])

I can make the Makefile end up in the right place, but autoconf decides
that @srcdir@ is '.' (rather than ..) and then eats my VPATH :-/

It would be really nice to have autoconf support such subdirectory builds
better, but at least VPATH should be preserved if it contains any colons
OR slashes. 

Rasmus

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