[cmake] rename nix lib to nixio

[gicmo]

Next step of the big rename (issue #595)

[jgrewe]

just wondered whether we also want to rename variables like nix_INCLUDES etc. ?

[achilleas-k]

Is renaming the variables necessary? I suppose there's a consistency argument for renaming them. How far would those changes go? Build scripts, etc?

[jgrewe]

I guess it is not necessary, my argument would be solely the consistency. The change should not affect anything else than the cmake file

[gicmo]

Renaming things sounds fine to me, but I would suggest we wait with it until all the functional changes are done (i.e. the lib rename, and the nix tool rename). So we separate the aesthetics from the semantics.