build: Update libtool files to 2.4.6
* m4/libtool.m4: Update from libtool-2.4.6, with modifications.
2008-04-06 Bruno Haible <bruno@clisp.org>
* m4/libtool.m4 (LT_INIT): When setting LIBTOOL, use
CONFIG_SHELL; don't assume that the Makefile.in will set SHELL
to ${CONFIG_SHELL}.
* m4/ltversion.m4: Likewise.
* build-aux/ltmain.sh: Likewise.
