autogen.sh: advice about installing libtool

autogen.sh: advice about installing libtool

libtool not being present in a system meant that this cryptic
error about LT_INIT would be shown:

sources/glue/Makefile.am:1: error: Libtool library used but 'LIBTOOL' is undefined
sources/glue/Makefile.am:1:   The usual way to define 'LIBTOOL' is to add 'LT_INIT'
sources/glue/Makefile.am:1:   to 'configure.ac' and run 'aclocal' and 'autoconf' again.
sources/glue/Makefile.am:1:   If 'LT_INIT' is in 'configure.ac', make sure
sources/glue/Makefile.am:1:   its definition is in aclocal's search path.
autoreconf: automake failed with exit status: 1