build: reconfigure automatically if configure.ac changes

build: reconfigure automatically if configure.ac changes

This provides a reduced but still useful sibling of the Automake's
"automatic Makefile rebuild" feature.  It's important to note that
we take care to enable the new rules only if the tree that has already
be configured with './configure', so that users relying on manual
configuration won't be negatively impacted.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>