examples: Support reading header comments from file.
This mirrors the Makefile.in.in change from 2015-09-01 with the amendment
from today.
* gettext-tools/examples/hello-*/po/Makefile.am ($(DOMAIN).pot-update):
Replace header entry with the content of $(DOMAIN).pot-header, if exists.
* gettext-tools/examples/hello-objc-gnustep/po/GNUmakefile: Likewise.
