examples: Use infrastructure from gettext version 0.20.
* gettext-tools/examples/hello-*/configure.ac: Use AM_GNU_GETTEXT_VERSION with
version 0.20.
* gettext-tools/examples/hello-c++-kde/configure.in.in: Likewise.
* gettext-tools/examples/hello-*/m4/Makefile.am: Update .m4 file list.
* gettext-tools/examples/hello-*/autoclean.sh: Likewise.
