Only set CMP0148 if known

Author: mbeards

rosidl_adapter_proto/cmake/rosidl_adapt_proto_interfaces.cmake

@@ -16,7 +16,10 @@
 #
 # ================================= Apache 2.0 =================================
 
-cmake_policy(SET CMP0148 OLD)
+if(POLICY CMP0148)
+  cmake_policy(SET CMP0148 OLD)
+endif()
+
 find_package(PythonInterp REQUIRED)
 if(NOT PYTHON_EXECUTABLE)
   message(FATAL_ERROR "Variable 'PYTHON_EXECUTABLE' must not be empty")