Link privately to protobuf static libraries (onnx#6881)

### Description
Link privately to protobuf static builds so that it doesn't need to find
protobuf.

Signed-off-by: cyy <cyyever@outlook.com>

Author: cyyever

CMakeLists.txt

@@ -260,7 +260,7 @@ if(NOT ONNX_PROTOC_EXECUTABLE)
     set(ABSL_PROPAGATE_CXX_STD 1)
     set(ONNX_BUILD_SHARED_LIBS ${BUILD_SHARED_LIBS})
     set(ONNX_CMAKE_CXX_FLAGS ${CMAKE_CXX_FLAGS})
-    # Use this setting to build thirdparty libs.
+    # Use this setting to build third-party libs.
     set(BUILD_SHARED_LIBS ${ONNX_USE_PROTOBUF_SHARED_LIBS})
     set(ProtobufURL https://github.com/protocolbuffers/protobuf/releases/download/v30.2/protobuf-30.2.tar.gz)
     set(ProtobufSHA1 f846d1f047255d09142361ba6d1985336db5e407)
@@ -474,7 +474,14 @@ if(ONNX_USE_LITE_PROTO)
     set(LINKED_PROTOBUF_TARGET protobuf::libprotobuf-lite)
   endif()
 endif()
-target_link_libraries(onnx_proto PUBLIC ${LINKED_PROTOBUF_TARGET})
+if((NOT ONNX_USE_PROTOBUF_SHARED_LIBS) AND Build_Protobuf)
+  target_link_libraries(onnx_proto PRIVATE ${LINKED_PROTOBUF_TARGET})
+  target_include_directories(onnx_proto PUBLIC
+    $<BUILD_INTERFACE:$<TARGET_PROPERTY:${LINKED_PROTOBUF_TARGET},INCLUDE_DIRECTORIES>>
+  )
+else()
+  target_link_libraries(onnx_proto PUBLIC ${LINKED_PROTOBUF_TARGET})
+endif()
 foreach(ABSL_USED_TARGET IN LISTS protobuf_ABSL_USED_TARGETS)
   if(TARGET ${ABSL_USED_TARGET})
     target_link_libraries(onnx_proto PRIVATE ${ABSL_USED_TARGET})

cmake/ONNXConfig.cmake.in

@@ -6,7 +6,9 @@
 # library version information
 set(ONNX_VERSION "@ONNX_VERSION@")
 
-find_package(Protobuf REQUIRED CONFIG)
+if((NOT @@ONNX_USE_PROTOBUF_SHARED_LIBS@@) AND @@Build_Protobuf@@)
+  find_package(Protobuf REQUIRED CONFIG)
+endif()
 
 # import targets
 include ("${CMAKE_CURRENT_LIST_DIR}/ONNXTargets.cmake")