ical.h always rebuilt

[Milan Crha]

	Hi,
while playing with the libical-glib integration I noticed that the
src/libical/ical.h is always rebuilt, unconditionally, which is not
necessary. Why it wasn't a big deal before, with the libical-glib
integration it means rebuild of the libical-glib library for both
`make` and `make install`.

The attached is a patch which makes the ical.h file rebuild only if
anything for it changed.
	Bye,
	Milan
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