Follow own advice about \ in shell double-quoted strings

* lib/autoconf/headers.m4 (_AC_HEADERS_EXPANSION):
* lib/m4sugar/m4sh.m4 (AS_ECHO):
Go back to using '%s\n' instead of "%s\n".
(AS_VAR_GET): Work around the quoting problem by using printf
directly with "%s\\n" instead of AS_ECHO; this is safe now that we
assume printf.