bootstrap: provide aclocal m4 from extras tools

The libtool.m4 file defining the LT_INIT macro is located in the share directory of the built libtool prefix. This file can be present on the host system but when it isn't, the bootstrap can fail during bootstrap with the following error:

autoreconf: running: automake --add-missing --copy --force-missing
bin/Makefile.am:41: error: Libtool library used but 'LIBTOOL' is undefined
bin/Makefile.am:41:   The usual way to define 'LIBTOOL' is to add 'LT_INIT'
bin/Makefile.am:41:   to 'configure.ac' and run 'aclocal' and 'autoconf' again.
bin/Makefile.am:41:   If 'LT_INIT' is in 'configure.ac', make sure
bin/Makefile.am:41:   its definition is in aclocal's search path.

This commit ensure that the libtool.m4 will be found in this case.