* Allow caching of external dependencies (everything but TBB, which I can't figure out yet)

* Tweaks to get CPU backend building and linking (but not actually working) on macOS

* Tweaks to codegen/CPU backend to enable macOS (no builtin headers yet)

* Re-enable header caching stuff on macOS

* Suppress unsupported compiler warning for Apple Clang

* Fix includes for macOS compatibility

(Squashing this commit but going to leave the full log here, unlike the way I usually do things)

* Incremental work on compatibility with system requirements

* Lower cmake version requirement so we can work with CentOS 7.4

* Disambiguate isnan call (CentOS's compiler doesn't like it)

* Make external_tbb conditional on NGRAPH_CPU_ENABLE

* Make MKLDNN inclusion conditional on NGRAPH_CPU_ENABLE

* Roll back desperation change to Ubuntu 14.04 binaries

* De-revert copyright date change (arrgh)

* Revert change of TBB's hard-coded compiler choice to gcc

* Cleanup of modified CMake stuff

* Add broken support for building LLVM from source

* Explain modification to json.hpp required compiler version

* Make TBB optional, and on by default. To disable, cmake -DNGRAPH_TBB_ENABLE=FALSE

* Add reasonable default for LLVM's CMAKE_ASM_COMPILER so users don't have to specify it

* Force TBB disabled if CMake is too old to build it (for CentOS)

* Build LLVM OpenMP as external project

* Tweak LLVM build options

* Minor comment edit in cmake scripts

* Workaround (fix?) for codegen crashes on CentOS

* Add some header paths for CentOS to resource_generator; temporary(?) tweak to ignore nonexistent paths

* Remove CMake compatibility figleaf since LLVM will require 3.1+ anyway

* Fix the TBB-enable flag to default to on

* Remove hard-coded choice of clang as tbb compiler

* ln -s -> ln -sf in command to create link for OpenMP source dir

* Make OpenMP build so it doesn't require symlink

* Allow use of prebuilt LLVM if requested at cmake time

* Some INSTALL instructions

* Allow NGRAPH_USE_PREBUILT_LLVM to be set in environment variable

* Tweak CI cmake options to use prebuilt LLVM

* Remove debug messages

* Rework defaulting behavior for LLVM_TARBALL_URL and LLVM_SHA1_HASH

* Remove warning message for unavailable header file directories

* fix resource generator dependencies

* add cr to end of file

* Support for MKLDNN convolution fprop in CPU backend based on Jai's implementation (no layout yet)

* CMake: TBB integration placeholder

* CMake: Integrate TBB

* CMake: Indent

* CMake: Rewrite TBB integration

* CMake: More TBB integration changes

* CMake: Install TBB headers and DSOs

* CMake: Don't install the TBB debug DSO

* CMake: Propagate ngraph's configured compiler setting over to MKL-DNN

* CMake: Restore TBB debug DSO installation

* CMake: Add installed headers to search path.
This needs to be cleaned up along with other header search cleanup

* CPU: Build and execute TBB flowgraphs

* CPU: TBB fixes

* CPU: More TBB fixes

* CPU: Allow both TBB and serial codegen for now

* TBB: get_arguments -> get_input_ops

* CPU: Use node methods

* CPU: Add TBB headers in the build directory to the search path

* TBB: Incorporate various changes from master

* CMake: Indentation fix

* CMake: Indentation fix

* CMake: TBB is mandatory so remove additional predicates

* TBB: Add a test

* CMake: Fix linker flags with GCC

* add resource file generator and store all headers used by codegen in memory.

* add json lib 2.1.1

* turn off clang-format for json lib

* add unit test file

* add code to poison the use of ngraph.hpp from within the ngraph library

* Done to achieve goal of having our #include directives all start
  with `"ngraph/`, like LLVM does.

add naming of tensors and tensor views
add visualization pass
add graph dump pass

* Implemented binops/unops with different return type from args

* Implemented generic bases class for comparison and arith ops, changed current comparison ops to subclass it

* Added a TraitedType for booleans

* Moved arith and comparison ops to separate header files from `op.h`

Start of external function calling support

Add memory layout and liveness passes.

add test framework

stuff wired up

benchmark segfault for 5M nodes. fun

make traverse_nodes not recursive

print timings on benchmark

unit test for pass manager

All tuples are expanded out to their component tensors.

Output: Describes one tensor output of an node
Input: Described one tensor input to a node, including the output supplying the value.

add cmake finder for graphviz dot program and then conditionally compile visualization based on finding

* Cleaned up the CMakeLists.txt to properly do the include paths and fixed the install targets.
Also cleaned up the header files located in sub directories.
* Added a simple C++ example with Makefile.
* UPdated the readme with TF usage instructions.

Element types needed some reorganization to allow literals to know what kind of values to hold.