gnulib-tool: Make copyright notice adjustment more robust.

gnulib-tool: Make copyright notice adjustment more robust.

* gnulib-tool (func_import): In sed_transform_main_lib_file,
sed_transform_build_aux_file, sed_transform_testsrelated_lib_file,
allow a line break to occur after "GNU" in "GNU [Lesser] General Public
License".
Reported by Glenn Morris <rgm@gnu.org> via Paul Eggert.