po, examples: Put stamp-po in the source directory.

po, examples: Put stamp-po in the source directory.

Rationale: <https://www.gnu.org/prep/standards/html_node/Makefile-Basics.html>.

* gettext-runtime/po/Makefile.in.in: Use $(srcdir)/stamp-po instead of stamp-po.
Talk about "version control system", not CVS.
* gettext-tools/examples/hello-*/po/Makefile.am: Likewise.
* gettext-tools/examples/po/Makefile.am: Likewise.
* gettext-tools/examples/check-examples (func_check_dist_vpath): Verify the
stamp-po file does not exist in the build dir, except when using the older
Makefile.in.in infrastructure.