Fix the bugs where libtoolize needs to use `dir/file' instead of

Fix the bugs where libtoolize needs to use `dir/file' instead of
`./dir/file', where ltdldir is `.', so that libtoolize correctly
checks for (and suggests) `config' and `m4' instead of
`./config' and `./m4' as auxiliary resp. macro directories.
The change is necessary for unambiguous naming, the chosen way
plays better with non-GNU make in VPATH builds.

* libtoolize.m4sh (ltdlprefix): New variable, to use as prefix
instead of `$ltdldir/'.
(func_check_macros): Use it.  Bug report by Eric Blake.