Test .cmake file to build a custom op

tf_user_op.cmake

# example how to build a user_op for windows
#
# 1. save this file as tensorflow/contrib/cmake/tf_user_ops.cmake
# 2. append "include(tf_user_ops.cmake)" to CMakeLists.txt
# 3. call cmake (see tensorflow/contrib/cmake/README.md), ie:
#     cmake %cmake_dir% -A x64 -DCMAKE_BUILD_TYPE=RelWithDebInfo -DPYTHON_EXECUTABLE=%PY%\python.exe 
#           -DPYTHON_LIBRARIES=%PY%\libs\python35.lib -DSWIG_EXECUTABLE=c:\local\swigwin-3.0.10\swig.exe 
#           -Dtensorflow_BUILD_PYTHON_TESTS=ON
# 4. you need to build the source tree once to generate all header files needed, ie:
#     MSBuild /p:Configuration=RelWithDebInfo tf_python_build_pip_package.vcxproj
# 5. build your user_ops
#     MSBuild /p:Configuration=RelWithDebInfo ackermann_ops.vcxproj
#    or if you know what you are doing you can skip checking dependencies:
#     MSBuild /p:Configuration=RelWithDebInfo  /p:BuildProjectReferences=false  ackermann_ops.vcxproj
#

set(tf_ackermanm_srcs
   "${tensorflow_source_dir}/tensorflow/core/user_ops/add_one.cc"
)
set(tf_ackermann_gpu_srcs
   "${tensorflow_source_dir}/tensorflow/core/user_ops/add_one_kernel.cu.cc"
)

# TODO: target_dir should be under ${CMAKE_CURRENT_BINARY_DIR}
set(target_dir
    "${tensorflow_source_dir}/tensorflow/core/user_ops"
)

AddUserOps(TARGET ackermann_ops
    SOURCES ${tf_ackermanm_srcs}
    GPUSOURCES ${tf_ackermann_gpu_srcs}
    DEPENDS pywrap_tensorflow_internal tf_python_ops
    DISTCOPY ${target_dir})

add_custom_command(TARGET ackermann_ops POST_BUILD
    COMMAND ${CMAKE_COMMAND} -E copy $<TARGET_FILE:ackermann_ops> "${target_dir}/ackermann_op.dll")