CMake: Update minimum CMake version to 3.0.2.

Previously, the minimum CMake version required was 2.6. This is very
old, and was not being tested. On further discussion (see
#214), it was noted that 3.0.2
is the oldest version mentioned in the current CMake documentation.

Signed-off-by: Ed Mooring <ed.mooring@xilinx.com>

CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required (VERSION 2.6)
+cmake_minimum_required (VERSION 3.0.2)
 if (POLICY CMP0048)
   cmake_policy(SET CMP0048 NEW)
 endif()