Libical: CMake version

[Milan Crha]

On Sat, 2016-12-17 at 09:23 -0500, Allen Winter wrote:
> For the next libical release I'd like to require cmake minimum
> version 3.1.0 (current is 2.8.9)

	Hi,
I'd like to mention that it's not an out of blue decision, it turned
out that CMake supports '-E env' only since 3.1.0. This had been added
in:
https://github.com/libical/libical/pull/246

> Does anyone object to this?

No objection here either, 3.1.0 is pretty old release (December 17th,
2014).
	Bye,
	Milan