Modern CMake usage: don't treat imported targets as files.

CMakeLists.txt

@@ -30,6 +30,10 @@ project(Magnum)
 if(POLICY CMP0042)
     cmake_policy(SET CMP0042 NEW)
 endif()
+# Don't treat imported targets with :: as files
+if(POLICY CMP0028)
+    cmake_policy(SET CMP0028 NEW)
+endif()
 
 set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${PROJECT_SOURCE_DIR}/modules/")