Fix portability regressions in today's earlier changeset.

Fix portability regressions in today's earlier changeset.

* libltdl/m4/libtool.m4 (_LT_PROG_XSI_REPLACE): `sed -i' is not
portable; use `mv -f ...|| cp -f ...' instead.
Add an initial backslash to sed `c' substitutions with leading
whitespace, so that indentation is not removed by some non-POSIX
compliant sed implementations.
(_LT_PROG_XSI_SHELLFNS): Display a diagnostic warning message if
one or more XSI function replacements failed.

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