Fix regression that broke Automake ‘make check’

Fix regression that broke Automake ‘make check’

Problem reported by Ken Moffat (sr#110287); the problem was
introduced in 2016-12-21T16:15:46Z!daniel.kitta@gmail.com.
* bin/autoheader.in (templates_for_header):
When generating warnings about symbols lacking templates,
downgrade template read failure from a fatal error to a warning.
Also, don’t even try to read from a template file whose name has
shell metavariables (which Autoconf 2.50 withdrew support for);
just warn about that, too.  These changes cause the Automake
tests to merely generate warnings that are ignored, instead
of failing.
* doc/autoconf.texi (Configuration Files, Configuration Headers)
(Configuration Commands, Configuration Links):
Also document here that the file names should not contain
shell metacharacters, to make this constraint more obvious.