Bump required CMake version to 2.8.

This is just a precautionary measure to avoid frustration on the
end user side, as older versions have not been tested in ages,
and will probably not work.

CMakeLists.txt

@@ -1,5 +1,5 @@
 project(ldc)
-cmake_minimum_required(VERSION 2.6)
+cmake_minimum_required(VERSION 2.8)
 
 set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_SOURCE_DIR}/cmake/Modules")