newlib: Remove automake option 'cygnus'

The 'cygnus' option was removed from automake 1.13 in 2012, so the
presence of this option prevents that or a later version of automake
being used.

A check-list of the effects of '--cygnus' from the automake 1.12
documentation, and steps taken (where possible) to preserve those
effects (See also this thread [1] for discussion on that):

[1] https://lists.gnu.org/archive/html/bug-automake/2012-03/msg00048.html

1. The foreign strictness is implied.

Already present in AM_INIT_AUTOMAKE in newlib/acinclude.m4

2. The options no-installinfo, no-dependencies and no-dist are implied.

Already present in AM_INIT_AUTOMAKE in newlib/acinclude.m4

Future work: Remove no-dependencies and any explicit header dependencies,
and use automatic dependency tracking instead.  Are there explicit rules
which are now redundant to removing no-installinfo and no-dist?

3. The macro AM_MAINTAINER_MODE is required.

Already present in newlib/acinclude.m4

Note that maintainer-mode is still disabled by default.

4. Info files are always created in the build directory, and not in the
source directory.

This appears to be an error in the automake documentation describing
'--cygnus' [2]. newlib's info files are generated in the source
directory, and no special steps are needed to keep doing that.

[2] https://lists.gnu.org/archive/html/bug-automake/2012-04/msg00028.html

5. texinfo.tex is not required if a Texinfo source file is specified.
(The assumption is that the file will be supplied, but in a place that
automake cannot find.)

This effect is overriden by an explicit setting of the TEXINFO_TEX
variable (the directory part of which is fed into texi2X via the
TEXINPUTS environment variable).

6. Certain tools will be searched for in the build tree as well as in the
user's PATH. These tools are runtest, expect, makeinfo and texi2dvi.

For obscure automake reasons, this effect of '--cygnus' is not active
for makeinfo in newlib's configury.

However, there appears to be top-level configury which selects in-tree
runtest, expect and makeinfo, if present. So, if that works as it
appears, this effect is preserved. If not, this may cause problem if
anyone is building those tools in-tree.

This effect is not preserved for texi2dvi. This may cause problems if
anyone is building texinfo in-tree.

If needed, explicit checks for those tools looking in places relative to
$(top_srcdir)/../ as well as in PATH could be added.

7. The check target doesn't depend on all.

This effect is not preseved. The check target now depends on the all
target.

This concern seems somewhat academic given the current state of the
testsuite.

Also note that this doesn't touch libgloss.

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