creating and using separate src and obj trees

[Simon Mudd]

I'm sure this is a faq: please point me to it!

I have been looking at the autoconf and automake manuals and on the
Internet
but have been unable to find out if I can use automake/autoconf to do this.
I've also searched on Google without success.

I have a source tree which is built using automake[1]. The application is
large
and being written in C++ and when built the tree ocupies over 1GB. I
therefore
want to separate the source tree from the build tree in a similar way that
FreeBSD uses /usr/src and /usr/obj when building the whole operating
system[2].

I'm not entirely sure if this question is more appropriate for the automake
list.

I've seen that there is a top_builddir and also an ac_top_builddir autoconf
macro/variable but am not sure how or what to set to enable me to define
this.
Ideally I would like to set top_buildir=$top_srcdir/../obj or similar and
have
this definition propogate down automatically in the subdirectories in my
source tree.

If anyone can point me to how to do this I would be most obliged.

[1][2] The source and object tree structure I am looking to achieve is:
src                 obj
src/lib             obj/lib
src/lib/lib1        obj/lib1
src/lib/lib2        obj/lib2
src/app1            obj/app1
src/app1/sub1       obj/app1/sub1
src/app1/sub2       obj/app1/sub2
src/app2            obj/app2
src/app3            obj/app3
...                 ...

Regards,

Simon Mudd