* lib/Automake/Variable.pm (require_variables): Mention aclocal's search path when suggesting to add a macro in configure.ac. People are confused when Automake suggests adding a macro (such as AC_PROG_LIBTOOL) that is already in configure.ac. * automake.in (require_file_internal): Suggest `automake --add-missing' for missing files that can be installed. * tests/library3.test: Adjust.