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

the LLVM integration requires tar.xz decompression support
which was added in 3.1

Conflicts:
	test/CMakeLists.txt
	test/execute.cpp

- Adds Sphinx docs support.

- Sphinx and Doxygen docs now built by CI.

- Sphinx and Doxygen docs now installed by `make install`.

This adds support for > < >= <= on bool, which by the type checker's standards should have been allowed anyway.

Also, this bug had presenting in a different way from expected (should have thrown "unhandled element type" from external_function, instead bonked because it incorrectly dispatched on int8_t). This was due to element::Type::operator== comparing only size, signedness, and floatiness, meaning that char and signed char (aka int8_t) look the same. Fixed by adding a test for m_cname equality.

Bob/test cleanup

make CMake report if gcc4.8 is being used

Remove commented-out code and predicate debuginfo generation

Also disable PCH in preparation of merge

via op::ParameterizedConstant