Improving prep_buildtree used in VPATH builds

Attached is the patch that tries to speedup prep_buildtree script, which is
used in VPATH builds, from our configure script.

The idea is to ask `find` to emit directory listing in depth-first order so
that the `mkdir -p` will create the deepest directory first and any
subsequent `mkdir -p` on an intermediate directory will not have to do
anything.

Currently I am seeing a performance improvement of this script by only about
500 ms; say 11.8 seconds vs. 11.3 secs. But I remember distinctly that
yesterday I was able to see an improvement of 11% on the same virtual
machine, averaged on multiple runs; 42 sec vs 37 sec. It might be the case
that the host OS or my Linux virtual machine were loaded at that time and
the filesystem could not cache enough inodes.

Seems like it would improve performance in general, but more so under load
conditions when you actually need it. I am not sure if -depth option is
supported by all incarnations of 'find'.

I have been away from Postgres development for quite a while, so would
appreciate if someone could tell me if such a patch should be submitted for
commitfest (since this is not actually a source patch).

Regards,
--
gurjeet.singh
@ EnterpriseDB - The Enterprise Postgres Company
http://www.EnterpriseDB.com

singh(dot)gurjeet(at){ gmail | yahoo }.com
Twitter/Skype: singh_gurjeet

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