Fix compilation with BUILD_SHARED_LIBS=ON on Windows

CMakeLists.txt

@@ -294,8 +294,8 @@ function(RELATIVE_PROTOBUF_GENERATE_CPP NAME SRCS HDRS ROOT_DIR DEPEND)
     return()
   endif()
 
-  # Add ONNX_API prefix to protobuf classes and methods in all cases
-  set(ONNX_DLLEXPORT_STR "dllexport_decl=ONNX_API:")
+  # Add ONNX_PROTO_API prefix to protobuf classes and methods in all cases
+  set(ONNX_DLLEXPORT_STR "dllexport_decl=ONNX_PROTO_API:")
 
   set(${SRCS})
   set(${HDRS})
@@ -489,17 +489,20 @@ if (MSVC)
     set(EXTRA_FLAGS "")
   endif()
   if (BUILD_SHARED_LIBS OR ONNX_BUILD_MAIN_LIB)
-    set(ONNX_API_DEFINE "-DONNX_API=__declspec(dllexport)")
+    set(ONNX_PROTO_API_DEFINE "-DONNX_PROTO_API=__declspec(dllexport)")
+    set(ONNX_PROTO_API_DEFINE_EXTERNAL "-DONNX_PROTO_API=__declspec(dllimport)")
   else()
-    set(ONNX_API_DEFINE "-DONNX_API=")
+    set(ONNX_PROTO_API_DEFINE "-DONNX_PROTO_API=")
   endif()
 else()
   # On non-Windows, hide all symbols we don't need
-  set(ONNX_API_DEFINE "-DONNX_API=__attribute__\(\(__visibility__\(\"default\"\)\)\)")
+  set(ONNX_PROTO_API_DEFINE "-DONNX_PROTO_API=__attribute__\(\(__visibility__\(\"default\"\)\)\)")
+  set(ONNX_PROTO_API_DEFINE_EXTERNAL "-DONNX_PROTO_API=__attribute__\(\(__visibility__\(\"default\"\)\)\)")
   set_target_properties(onnx_proto PROPERTIES CXX_VISIBILITY_PRESET hidden)
   set_target_properties(onnx_proto PROPERTIES VISIBILITY_INLINES_HIDDEN 1)
 endif()
-target_compile_definitions(onnx_proto PRIVATE ${ONNX_API_DEFINE})
+target_compile_definitions(onnx_proto PRIVATE ${ONNX_PROTO_API_DEFINE})
+target_compile_definitions(onnx_proto INTERFACE ${ONNX_PROTO_API_DEFINE_EXTERNAL})
 
 if(ONNX_USE_LITE_PROTO)
   if(TARGET protobuf::libprotobuf-lite)