examples: hello-c++-widgets: Update build infrastructure.
Gnulib changed on 2018-11-11: asm-underscore.m4 is no longer needed for havelib.
* gettext-tools/examples/hello-c++-widgets/autogen.sh: Don't pull in
asm-underscore.m4.
* gettext-tools/examples/hello-c++-widgets/autoclean.sh: Don't remove
asm-underscore.m4.
