* Cleanup how compile flags set and used by nGraph and external projects.
Set C++11 through CMake and pass it down to external projects.
Prefer CMake variables such as CMAKE_POSITION_INDEPENDENT_CODE and
CMAKE_CXX_STANDARD instead of explicitly setting compiler dependent
flags.
Create json compilation database for external projects.
CMAKE_CXX_FLAGS is used as common global options for nGraph and external
projects.
add_compile_options() is used for local options for current and sub
directories.
add_definitions() is used for setting definitions for current and sub
directories.
Note: Global options are not passed down to some external projects.
Note: mkl-dnn resets CMAKE_CXX_FLAGS internally.
Note: TBB and MLSL are not CMake based.
Noet: Eigen and json is header only library.

* Fix error.

* Fix error. (second attempt)

* Cleanup code.

* Allow check for undefined macro.

* Try to fix cldnn issue.

* Set type for CMake arguments.

* Pass C++ standard to protobuf.

* Pass C++ standard down to TBB.

* Change how Clang specific flags are handled.

* Fix error.

* Workaround for compile error on Baidu's PDPD docker.

* Fix windows build error.

* Use git shallow clone if CMake >= 3.6. Remove BUILD_BYPRODUCTS.

* Fix ONNX, protobuf integration.

* Use uniform naming for external project prefixes and use INSTALL_DIR instead of SOURCE_DIR for referencing prebuilt external libraries.

* TBB: Use single variable to share path across files.

* update licenses for 2019

* style

* Forward nGraph's C/C++ compiler, build type and generator information to cmake based external projects.

* Fix typo.

* Pass generator related info properly.

* Googletest is using DEBUG_POSTFIX

* TBB uses postfix for debug lib.

* Fix typo.

* update build byproducts to support ninja

* remove unused cmake code

* more cmake cleanup

* display error message if Ninja generator is requested

* fix mkldnn ext project

* revert onnx cmake file

* revert protobuf cmake file

* revert mlsl cmake file

* more fixing

* Add missing halide dependency

* Bug fix in halide op creation

* Localize halide/llvm to cpu backend

* Added comments

* Pass NGRAPH_HALIDE to tests

* Resolve merge conflicts