Git bug. "make [all]" does not use USE_LIBPCRE when "configure --with-libpcre" was previously run

Hello, Stefan!

I'll begin from afar.

Today I was suprised when that command gave me the error
    "fatal: cannot use Perl-compatible regexes when not compiled with
    USE_LIBPCRE"

As far as I always use
    ./configure --with-libpcre 
    make all
to build my Cygwin version of Git that was a VERY sudden error for me.
(but this is not a Cygwin-specific error, I've reproduced it on my Linux
box).

I've found out that `make` "does not know" anymore about USE_LIBPCRE=YesPlease even `./configure --with-libpcre` was run before it. But I do remember that it worked in some versions before (currently I use latest - v1.8.3.4).

So, I've got armed with the following test script:

---8<--- #!/bin/sh

set -e
./configure --with-libpcre
make
set +e
./bin-wrappers/git grep -qIP das+
# if "cannot use Perl-compatible regexes" error occurs
# then Git exits with an error code 128 (but `git bisect run` fails to
# expect this exit code).
if test $? -eq 128
then
    exit 1
fi
---8<---

and `git bisect run` to find out where Git was broken. That gave me:

---8<---
40bfbde9da5c6cbc85f49a755f27162dc966fd89 is the first bad commit
commit 40bfbde9da5c6cbc85f49a755f27162dc966fd89
Author: Stefano Lattarini <stefano.lattarini@gmail.com>
Date:   Tue Sep 11 17:45:30 2012 +0200

    build: don't duplicate substitution of make variables
    
    Thanks to our 'GIT_CONF_SUBST' layer in configure.ac, a make variable 'VAR'
    can be defined to a value 'VAL' at ./configure runtime in our build system
    simply by using "GIT_CONF_SUBST([VAR], [VAL])" in configure.ac, rather than
    having both to call "AC_SUBST([VAR], [VAL])" in configure.ac and adding the
    'VAR = @VAR@' definition in config.mak.in.  Less duplication, less margin
    for error, less possibility of confusion.
    
    While at it, fix some formatting issues in configure.ac that unnecessarily
    obscured the code flow.
    
    Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>
    Signed-off-by: Junio C Hamano <gitster@pobox.com>

:100644 100644 802d34223a2859ee1341d94ee722d7939b7276aa 69d48382fe69b8699eb350949fff04975db923f8 M config.mak.in :100644 100644 450bbe7f1020711b4af2ad6ea52d717651c30b0b da1f41f58871b2102a9ed5eaeff7df3c9623f4bd M configure.ac bisect run success

---8<---

I'm not a C programmer to fix that, so I ask you to.