examples: Avoid grepping through monster files.
This mirrors the Makefile.in.in change from 2010-06-12.
Reported by Jim Meyering <jim@meyering.net>.
* gettext-tools/examples/hello-*/po/Makefile.am (($(DOMAIN).pot-update): When
GNU find is available, avoid grepping through monster files.
* gettext-tools/examples/hello-objc-gnustep/po/GNUmakefile: Likewise.
