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>