Emit compat shims less often for old AS_ECHO implementation details.

Emit compat shims less often for old AS_ECHO implementation details.

The shell variables $as_echo and $as_echo_n and the substitution
variables @ECHO_C@, @ECHO_N@, and @ECHO_T@ are implementation
details of the old implementation of AS_ECHO and AS_ECHO_N
(2.69-era; before the switch to use ‘printf’).  We continue
to provide them for compatibility with various third-party macros and
Makefiles that used them, even though, as far as I can tell, they were
never documented.  (Someone made ECHO_C etc. substitution variables on
purpose, though.)

For $as_echo and $as_echo_n, we can tell precisely when they are
needed, because all uses are exposed to M4 macro expansion.
Therefore, emit the necessary support code on first use and not
from AS_INIT.  As a side effect this means the -Wobsolete warning
only appears on the first use of each, which IMHO is polite.

For @ECHO_C@, @ECHO_N@, and @ECHO_T@, we have no way of knowing
whether a substitution variable is used, but we *do* know that
substitution variables are exclusively a configure.ac thing, so their
support code can be moved to general.m4 and emitted from AC_INIT
rather than from AS_INIT.

(It’s faintly possible that someone decided to use $ECHO_C in an
Autotest script but considering how rare it is for people to use
Autotest at all, or any other non-Autoconf application of M4sh,
I think we can worry about that when we hear about it.)

* lib/m4sugar/m4sh.m4 (_AS_ECHO_N_PREPARE): Removed.
  (AS_PREPARE, _AS_PREPARE): Don’t use _AS_ECHO_N_PREPARE.
  (_as_echo_var_prepare, _as_echo_n_var_prepare): New macros which
  emit the -Wobsolete warning and the support code for $as_echo and
  $as_echo_n respectively.
  (as_echo, as_echo_n): Move to bottom of the block that sets up
  the current implementation of AS_ECHO.  Invoke _as_echo_var_prepare
  and _as_echo_n_var_prepare, respectively, on first use.

* lib/autoconf/general.m4 (_AC_INIT_ECHO_N): New internal macro,
  contains the probe code that used to be in _AS_ECHO_N_PREPARE.
  AC_SUBSTs ECHO_C, ECHO_N, and ECHO_T.
  (AC_INIT): Don’t AC_SUBST ECHO_C, ECHO_N, and ECHO_T here.
  Invoke _AC_INIT_ECHO_N.