config.status is careful not to update any CONFIG_HEADERs (config.h or
whatever) unless they would actually change. It would be extremely
useful if this behaviour applied to all output files (such as
Makefiles), so sophisticated build systems don't rebuild thing
unnecessarily.

As a case in point, touching configure.ac causes the entire tree to be
rebuilt when using automake (and any similarly correct set of
makefiles).