Make 'make distcheck' work in a VPATH build.

Make 'make distcheck' work in a VPATH build.

* Makefile.am (distcheck-hook): Compare files in $(srcdir).
* gettext-tools/examples/po/xsmallpot.sh: Put the resulting POT file in the
source dir, not in the build dir.
* gettext-tools/examples/po/Makefile.am (update-po): Execute sub-make in the
build dir, not in the source dir.
(SMALLPOFILES_FOR_lang): Prefix file names with $(srcdir).
(hello-*/po/*.po): Likewise. Execute mmsmallpo.sh in $(srcdir).