Fix the way we find protobuf library (onnx#539)

* Fix the way we find protobuf library

* Review comments

* Fix

* indent

* Fix build

* Reset changes on setup.py

CMakeLists.txt

@@ -16,7 +16,44 @@ if (WIN32)
     set(CMAKE_STATIC_LINKER_FLAGS "${CMAKE_STATIC_LINKER_FLAGS} /WX")
 endif()
 add_subdirectory(third_party/pybind11)
-FIND_PACKAGE(Protobuf REQUIRED)
+
+# Customized version of find Protobuf. We need to avoid situations mentioned
+# in https://github.com/caffe2/caffe2/blob/b7d983f255ef5496474f1ea188edb5e0ac442761/cmake/ProtoBuf.cmake#L82-L92
+# The following section is stolen from cmake/ProtoBuf.cmake in Caffe2
+find_program(PROTOBUF_PROTOC_EXECUTABLE
+  NAMES protoc
+  DOC "The Google Protocol Buffers Compiler")
+
+# Only if protoc was found, seed the include directories and libraries.
+# We assume that protoc is installed at PREFIX/bin.
+# We use get_filename_component to resolve PREFIX.
+if(PROTOBUF_PROTOC_EXECUTABLE)
+  get_filename_component(
+    _PROTOBUF_INSTALL_PREFIX
+    ${PROTOBUF_PROTOC_EXECUTABLE}
+    DIRECTORY)
+  get_filename_component(
+    _PROTOBUF_INSTALL_PREFIX
+    ${_PROTOBUF_INSTALL_PREFIX}/..
+    REALPATH)
+  find_library(PROTOBUF_LIBRARY
+    NAMES protobuf
+    PATHS ${_PROTOBUF_INSTALL_PREFIX}/lib
+    NO_DEFAULT_PATH)
+  find_library(PROTOBUF_PROTOC_LIBRARY
+    NAMES protoc
+    PATHS ${_PROTOBUF_INSTALL_PREFIX}/lib
+    NO_DEFAULT_PATH)
+  find_library(PROTOBUF_LITE_LIBRARY
+    NAMES protobuf-lite
+    PATHS ${_PROTOBUF_INSTALL_PREFIX}/lib
+    NO_DEFAULT_PATH)
+  find_path(PROTOBUF_INCLUDE_DIR
+    google/protobuf/service.h
+    PATHS ${_PROTOBUF_INSTALL_PREFIX}/include
+    NO_DEFAULT_PATH)
+  find_package(Protobuf REQUIRED)
+endif()
 
 # Build the libraries with -fPIC
 set(CMAKE_POSITION_INDEPENDENT_CODE ON)