maint: pick XSI funcs at runtime, not configure time.

maint: pick XSI funcs at runtime, not configure time.

Determine, on a function by function basis, what XSI features
are available in the shell that is actually running the script,
rather than the one that was picked at configure time by the
re-execution engine.
* m4/libtool.m4 (_LT_PROG_FUNCTION_REPLACE)
(_LT_PROG_REPLACE_SHELLFNS): Remove.
(_LT_CHECK_SHELL_FEATURES): Remove tests for XSI and += support.
* build-aux/general.m4sh (func_append, func_append_quoted)
(func_arith, func_len, func_basename, func_dirname)
(func_dirname_and_basename, func_stripname): List both enhanced
and fallback implementations, and choose one based on whether
the enhanced features required by the fastest version are
available.
* build-aux/getopt.m4sh (func_split_short_opt)
(func_split_long_opt): Ditto.
* build-aux/ltmain.m4sh (func_lo2o, func_xform): Ditto.
* tests/getopt-m4sh.at: Add tests for enhanced and fallback
implementations of func_arith, func_len, func_stripname and
func_dirname_and_basename.
* NEWS: Updated.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>