Arrange so that onnx-optimizer (1) uses our own ONNX build, 
(2) builds as a shared library, and (3) links against the shared
libraries of ONNX.

diff --git a/CMakeLists.txt b/CMakeLists.txt
index c2e48b35..8af51076 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -9,8 +9,6 @@ endif(NOT MSVC)
 
 set(CMAKE_POSITION_INDEPENDENT_CODE ON)
 
-set(ONNX_ROOT ${PROJECT_SOURCE_DIR}/third_party/onnx)
-add_subdirectory(${ONNX_ROOT})
 
 file(READ "${PROJECT_SOURCE_DIR}/VERSION_NUMBER" ONNX_OPTIMIZER_VERSION)
 string(STRIP "${ONNX_OPTIMIZER_VERSION}" ONNX_OPTIMIZER_VERSION)
@@ -21,14 +19,18 @@ file(GLOB_RECURSE onnx_opt_srcs "onnxoptimizer/*.cc"
 list(REMOVE_ITEM onnx_opt_srcs "${PROJECT_SOURCE_DIR}/onnxoptimizer/cpp2py_export.cc")
 
 add_library(onnx_optimizer ${onnx_opt_srcs})
-target_link_libraries(onnx_optimizer PUBLIC onnx)
+target_link_libraries(onnx_optimizer PUBLIC onnx onnx_proto)
 target_include_directories(onnx_optimizer PUBLIC
     $<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}>
     $<INSTALL_INTERFACE:include>
     )
 
+# These cpp macros must be defined so the ONNX headers behave
+# correctly.
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DONNX_ML=1 -DONNX_NAMESPACE=onnx")
+
 add_executable(onnx_optimizer_exec examples/onnx_optimizer_exec.cpp)
-target_link_libraries(onnx_optimizer_exec onnx_optimizer)
+target_link_libraries(onnx_optimizer_exec onnx_optimizer protobuf)
 
 if(BUILD_ONNX_PYTHON)
   if("${PY_EXT_SUFFIX}" STREQUAL "")
@@ -79,11 +81,10 @@ if(BUILD_ONNX_PYTHON)
                           PRIVATE $<TARGET_OBJECTS:onnx_optimizer>)
   else()
     # Assume everything else is like gcc
-    target_link_libraries(onnx_opt_cpp2py_export
-                          PRIVATE "-Wl,--whole-archive" $<TARGET_FILE:onnx_optimizer>
-                                  "-Wl,--no-whole-archive")
+    target_link_libraries(onnx_opt_cpp2py_export PRIVATE onnx_optimizer)
     set_target_properties(onnx_opt_cpp2py_export
-                          PROPERTIES LINK_FLAGS "-Wl,--exclude-libs,ALL")
+                          PROPERTIES LINK_FLAGS
+			  "-Wl,-rpath=${CMAKE_INSTALL_PREFIX}/lib")
   endif()
 
   target_link_libraries(onnx_opt_cpp2py_export PRIVATE onnx_optimizer)