and remove need for the dso_handle symbol

NGRAPH_CPU_ENABLE

This will be revisited if a performant tanh is needed

cmake now fails if zlib is not found

CPU Backend

Conflicts:
	src/ngraph/codegen/compiler.cpp

the other backend flag

The CPU backend is disabled by default and must be enabled with
the CMake variable NGRAPH_CPU_ENABLE set to true

Conflicts:
	src/ngraph/runtime/cpu/call_frame.cpp
	src/ngraph/runtime/cpu/call_frame.hpp
	src/ngraph/runtime/cpu/external_function.cpp
	test/cpu.cpp

Fix bug with multiple updates to adjoint map