Check and enforce CMake version

CMakeLists.txt

@@ -186,6 +186,7 @@ project(px4 CXX C ASM)
 if (NOT ${CMAKE_VERSION} VERSION_LESS 3.0.0)
 	cmake_policy(SET CMP0045 NEW) # error on non-existent target in get prop
 	cmake_policy(SET CMP0046 NEW) # no non-existent targets as dependencies
+	cmake_policy(SET CMP0025 OLD) # still report AppleClang as Clang
 endif()
 if (NOT ${CMAKE_VERSION} VERSION_LESS 3.1.0)
 	cmake_policy(SET CMP0054 NEW) # don't dereference quoted variables