Add ICAL_CHECK_VERSION macro

[Milan Crha]

	Hello,
looking into the icalversion.h.cmake, it's not that useful to have the
version defined as a string. To be honest, I do not know whether it can
be used for conditional compiles.

Due to that, it would be handy to introduce a version check macro like
some other libraries do. I would name it:
   ICAL_CHECK_VERSION(major,minor,micro)
You can copy&paste most of the things from gtk+, for example:
https://git.gnome.org/browse/gtk+/tree/gtk/gtkversion.h.in
thus it would be pretty simple to do.

I also noticed that pkg-config returns only major.minor and ignores
micro, similar with the ICAL_VERSION from icalversion.h. Is it
intentional? Why to hide the micro version?

	Thanks and bye,
	Milan