examples: Add support for msgmerge --previous.
This mirrors the Makefile.in.in change from 2016-07-01.
* gettext-tools/examples/hello-*/po/Makefile.am ($(POFILES), update-po): Use
option --previous when the msgmerge program supports it.
* gettext-tools/examples/hello-objc-gnustep/po/GNUmakefile: Likewise.
