COMP: Update CMake required version from 2.8.9 to 3.0 to match its use

Following 6e805ec (ENH: Export CTK_USE_QVTKOPENGLWIDGET definition), the
use of target_compile_definitions() was introduced.

Since this CMake command is only available with CMake >= 3.0, this commit
updates the CMake minimum required version.

In addition of keeping things simple (e.g avoid requiring a newer CMake
version only if specific set of option is enabled), this commit also make it
possible to use of commands for specifying "usage requirements" [1] of CTK
targets unconditionally.

[1] https://cmake.org/cmake/help/v3.0/manual/cmake-buildsystem.7.html#build-specification-and-usage-requirements

CMakeLists.txt

@@ -18,7 +18,7 @@
 #
 ###########################################################################
 
-cmake_minimum_required(VERSION 2.8.9)
+cmake_minimum_required(VERSION 3.0)
 
 foreach(p
   CMP0054 # CMake 3.1