Add our own version of libtool.m4 instead of relying on the installed one.

This is a potential version mismatch problem because this file came from
libtool 1.5.10. If you're running a different version of libtool, the
checks in this file may not be quite right. Having our own version of this
file violates the spirit of libtool and is only provided to change the name
of "libtool" to "mklib". This is done so that the name "libtool" does not
conflict with the "lib" directory when doing tab completion and your
$objdir == $srcdir.

llvm-svn: 16411