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>

Makefile

@@ -2167,8 +2167,14 @@ configure: configure.ac GIT-VERSION-FILE
 	$(RM) $<+
 
 ifdef AUTOCONFIGURED
-config.status: configure
-	$(QUIET_GEN)if test -f config.status; then \
+# We avoid depending on 'configure' here, because it gets rebuilt
+# every time GIT-VERSION-FILE is modified, only to update the embedded
+# version number string, which config.status does not care about.  We
+# do want to recheck when the platform/environment detection logic
+# changes, hence this depends on configure.ac.
+config.status: configure.ac
+	$(QUIET_GEN)$(MAKE) configure && \
+	if test -f config.status; then \
 	  ./config.status --recheck; \
 	else \
 	  ./configure; \