Introduce @unless/@endunless and postbootstrap Makefile targets

This patch turns dependencies of non-bootstrap targets on bootstrap
targets for bootstrap builds into dependencies on stage_last.  This
arrangement gets stage1-bubble to run from stage_last if we haven't
started a bootstrap yet, and to use the current stage otherwise.  This
was already the case of target libs, just not of non-bootstrapped host
modules.

In order to retain preexisting dependencies in non-bootstrap builds,
or in gcc-less builds, this introduces support for @unless/@endunless
pairs in Makefile.in.

There is a remaining possibility of problem if activating, in a tree
configured for bootstrap, a parallel build of two or more modules, at
least one bootstrapped and one not.  In this case, make might decide
to build stage_current and stage_last in parallel, the latter will
start a submake to build stage1 while the initial make, having
satisfied stage_current, proceeds to build the bootstrapped module in
non-bootstrapped configurations.  The two builds will overlap and will
likely conflict.  This situation does NOT arise in normal settings,
however: a post-bootstrap build of all-host all-target will indeed
activate such targets concurrently, but only after building all
bootstrapped modules successfully, and it will have both stage_last
and stage_current targets already satisfied, so the potential race
between builds will not arise.

Another remaining problem, that is slightly expanded with this patch,
is that of an interrupted build in a tree configured for bootstrap,
continued with a non-bootstrapped target.  Target modules that were
not bootstrapped would already fail to complete the current stage when
activated explicitly in the command line for a retry; host modules,
however, would attempt to build their bootstrapped dependencies, which
is what led to the problem of concurrent builds addressed with this
patch.  An interrupted or failed build might still recover correctly,
if the non-bootstrapped target is activated in both builds, because
then make will remove stage_last when its build command is
interrupted, so that it will attempt to recreate it with stage1-bubble
in the second try.  A bootstrap build, however, will not be attempting
to build stage_last, so the file will remain and the retry won't go
through stage1-bubble.  We have lived with that for target modules, so
we can probably live with that for host modules too.

Another undesirable consequence of this change is that non-boostrapped
host modules, in a tree configured for bootstrap, when activated as
make all-<module>, will build all of stage1 instead of only the
module's usual dependencies.  This is intentional and necessary to fix
the parallel-build problem.  If it's not desirable, disabling the
unnecessary bootstrap configuration will suffice to restore the
original set of dependencies.

for  ChangeLog

	* configure.ac: Introduce support for @unless/@endunless.
	* Makefile.tpl (dep-kind): Rewrite with cond; return
	postbootstrap in some cases.
	(make-postboot-dep, postboot-targets): New.
	(dependencies): Do not output postbootstrap dependencies at
	first.  Output non-target ones changed for configure to depend
	on stage_last @if gcc-bootstrap, and the original deps @unless
	gcc-bootstrap.
	* configure.in, Makefile.in: Rebuilt.