[ng] built sources: avoid unconditional recursive make invocation

With this change, the times for a null-build of GNU coreutils (as
averaged from ten builds on an aging Debian system) drops from
~3.8 seconds to ~2.6 seconds.

With this change, the semantic of $(BUILT_SOURCES) support is slightly
altered, in that $(BUILT_SOURCES) will now be built not only before
the "all", "check" or "install" targets, but before *any* target.

We believe that not only this change in semantics is justified by
the enhanced performance, but that the new semantics is actually
better than the old one.  So a double win.  The new and more complex
implementation is a price worth paying for those improvements.

* lib/am/all-target.am: New implementation of $(BUILT_SOURCES) support,
using more GNU make features (mostly, automatic remake of include
files and automatic restart in the face of rebuilt makefiles).  This
new implementation should ensure that $(BUILT_SOURCES) will be built
before before any other target.
* lib/am/check-target.am, lib/am/install.am: Simplify: now there's
no need to explicitly cater for $(BUILT_SOURCES) in the 'check' and
'install' targets.
* t/built-sources-subdir.sh: Adjust and enhance a little.
* t/remake-gnulib-remove-header.sh: Trivial adjustment (relying on the
fact that .PHONY works correctly for GNU make, also when applied to
targets that are existing files).
* t/suffix-hdr.sh: Adjust, to avoid GNU make erroneously remove a
generated header thinking it is an intermediate file.
* t/yacc-deleted-headers.sh: Adjust: we should expect non-existing
$(BUILT_SOURCES) to be rebuilt by *any* make invocation.
* t/yacc-mix-c-cxx.sh: Likewise.
* NG-NEWS (Source Files with Unknown Extensions): Adjust the example
to mirror the change.
(Miscellaneous): Document the new $(BUILT_SOURCES) semantic.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>

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