add_opp_target: force build order

Author: AshFungor

cmake/GenerateOppMessage.cmake

@@ -29,4 +29,9 @@ function(generate_opp_message msg_input)
 
     target_sources(${args_TARGET} PRIVATE "${msg_output_source}" "${msg_output_header}")
     target_include_directories(${args_TARGET} PUBLIC ${msg_output_root})
+
+    # force cmake to generate message first, in case actual args_TARGET
+    # sources use results from custom_command (include message headers)
+    add_custom_target(dummy_${msg_name}_target DEPENDS "${msg_output_header}")
+    add_dependencies(${args_TARGET} dummy_${msg_name}_target)
 endfunction()