Implement $ECHO with print -r, printf or cat in a shell function.

Implement $ECHO with print -r, printf or cat in a shell function.

* configure.ac: Bump Autoconf requirement.
* libltdl/config/general.m4sh: Use $as_echo as default $ECHO.
* tests/testsuite.at: Use $as_echo as default $ECHO.

* libltdl/m4/libtool.m4 (LT_INIT): Add _LT_SHELL_INIT to
work around Autoconf <2.64 bug.
(_LT_OUTPUT_LIBTOOL_COMMANDS_INIT): Add func_fallback_echo.
Eliminate lt_ECHO requoting.
(_LT_SHELL_INIT): Use a public M4sh diversion.
(_LT_PROG_ECHO_BACKSLASH): Rewrite.
(LT_CMD_MAX_LEN): Do not use --fallback-echo.
* libltdl/config/ltmain.m4sh: Remove --no-reexec and --fallback-echo
handling.
(func_fallback_echo): New.
(func_emit_wrapper_part1): Quote ECHO.  Remove --no-reexec and
--fallback-echo handling.
(Execute mode): Do not set qecho.