doc: modernize character encoding

* doc/autoconf.texi: Specify @documentencoding UTF-8.  Don't abuse
'`' to mean open quote, unless Texinfo already interprets it that
way.  Be more careful about hyphen versus minus versus endash
versus emdash.
doc: Solaris 11 supports $(...)

* doc/autoconf.texi (Shell Substitutions): Say that $(...) isn't
working in Solaris 10 and earlier, not in "recent releases".
autoconf: modernize AC_C_VARARRAYS for C11

* lib/autoconf/c.m4 (AC_C_VARARRAYS): Define __STDC_NO_VLA__ if
VLAs are not supported, as this is what C11 does.  The old macro
HAVE_C_VARARRAYS is still defined if they are supported, but is
now obsolescent.  Also, check for VLA bug in GCC 3.4.3.
* doc/autoconf.texi (C Compiler), NEWS: Document the above.