Optimize clients of AS_REQUIRE.

Optimize clients of AS_REQUIRE.

* lib/m4sugar/m4sugar.m4 (m4_defun): Add undocumented third
argument.
(m4_defun_init): New undocumented macro.
* lib/m4sugar/m4sh.m4 (_AS_ECHO_LOG, AS_MESSAGE, AS_BASENAME)
(_AS_DIRNAME_EXPR, AS_DIRNAME, AS_ECHO, AS_ECHO_N, AS_TEST_X)
(AS_LN_S, AS_MKDIR_P, _AS_PATH_WALK, AS_VERSION_COMPARE)
(AS_TR_SH, AS_TR_CPP, AS_VAR_APPEND, AS_VAR_PUSHDEF): Use it to
simplify these macros once the one-shot initialization is
complete.
* tests/m4sugar.at (m4@&t@_require: one-shot initialization): New
test.

Signed-off-by: Eric Blake <ebb9@byu.net>