build: do not automatically reconfigure unless configure.ac changed

build: do not automatically reconfigure unless configure.ac changed

Starting with v1.7.12-rc0~4^2 (build: reconfigure automatically if
configure.ac changes, 2012-07-19), "config.status --recheck" is
automatically run every time the "configure" script changes.  In
particular, that means the configuration procedure repeats whenever
the version number changes (since the configure script changes to
support "./configure --version" and "./configure --help"), making
bisecting painfully slow.

The intent was to make the reconfiguration process only trigger for
changes to configure.ac's logic.  Tweak the Makefile rule to match
that intent by depending on configure.ac instead of configure.

Reported-by: Martin von Zweigbergk <martinvonz@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Reviewed-by: Jeff King <peff@peff.net>
Reviewed-by: Stefano Lattarini <stefano.lattarini@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>