m4sugar: fix AS_VAR_GET regression.

m4sugar: fix AS_VAR_GET regression.

AS_VAR_GET expands AS_ECHO inside en evaled single quoted string,
which causes the single quotes in "printf '%s\n'" to expose the
%s\n to the shell which expands "\n" to simply "n" before passing
it to printf.
* lib/m4sugar/m4sh.m4 (AS_ECHO): Use double quotes around the
format string.
* doc/autoconf.texi (Limitations of Shell Builtins): Show double
quotes to match AS_ECHO expansion.
* NEWS: Likewise.