build: Read POTFILES.in at 'make' time, not at 'configure' time.

build: Read POTFILES.in at 'make' time, not at 'configure' time.

* gettext-runtime/m4/po.m4 (AM_PO_SUBDIRS): Bump GETTEXT_MACRO_VERSION to 0.24.
Don't create POTFILES any more.
(AM_POSTPROCESS_PO_MAKEFILE): Don't substitute POTFILES_DEPS any more.
* gettext-runtime/po/Makefile.in.in (GETTEXT_MACRO_VERSION): Bump to 0.24.
Mention 'make' requirements.
(POTFILES, POTFILES_DEPS): Define with a computed value.
($(DOMAIN).pot-update): Depend on POTFILES_DEPS, not POTFILES.
(distclean): Don't remove POTFILES any more.
* gettext-tools/examples/README: Mention 'make' requirements.
* gettext-tools/examples/po/Makefile.am (POTFILES_DEPS): Define with a computed
value.
* gettext-tools/examples/hello-*/po/Makefile.am (POTFILES_DEPS): Likewise.