Define $as_echo and $as_echo_n for backward compatibility.

Define $as_echo and $as_echo_n for backward compatibility.

Commit 2b59b6f8a79b8bf77e178ff4e5aa0ede433d39cf removed the internal
shell variables $as_echo and $as_echo_n.  It turns out that these are
used by several widely-used third-party m4 files (notably both
gnulib-common.m4 from gnulib, and ax_pthread.m4 from the Autoconf
macro archive) as well as any number of existing configure.ac’s.

Restore these shell variables, unconditionally defining them to use
printf.  Issue -Wobsolete warnings if they are used, recommending the
use of AS_ECHO and AS_ECHO_N respectively.  Add a test which checks
both that they do work and that they trigger warnings.