Fix CMake proto_path

cmake/cpp.cmake

@@ -100,6 +100,15 @@ add_library(${PROJECT_NAME}::${PROJECT_NAME} ALIAS ${PROJECT_NAME})
 set(PROTO_HDRS)
 set(PROTO_SRCS)
 file(GLOB_RECURSE proto_files RELATIVE ${PROJECT_SOURCE_DIR} "ortools/*.proto")
+
+# Get Protobuf include dir
+get_target_property(protobuf_dirs protobuf::libprotobuf INTERFACE_INCLUDE_DIRECTORIES)
+foreach(dir ${protobuf_dirs})
+	if ("${dir}" MATCHES "BUILD_INTERFACE")
+		list(APPEND PROTO_DIRS "--proto_path=${dir}")
+  endif()
+endforeach()
+
 foreach (PROTO_FILE ${proto_files})
 	#message(STATUS "protoc proto(cc): ${PROTO_FILE}")
 	get_filename_component(PROTO_DIR ${PROTO_FILE} DIRECTORY)
@@ -112,6 +121,7 @@ foreach (PROTO_FILE ${proto_files})
 		OUTPUT ${PROTO_SRC} ${PROTO_HDR}
 		COMMAND protobuf::protoc
 		"--proto_path=${PROJECT_SOURCE_DIR}"
+		"${PROTO_DIRS}"
 		"--cpp_out=${PROJECT_BINARY_DIR}"
 		${PROTO_FILE}
 		DEPENDS ${PROTO_FILE} protobuf::protoc