when building tests with both, Boost.Build and CMake.
Disable concepts check for tests where range (e.g. std::array)
used as image pixel - not fully specialised as acceptable pixel type.
Ignore warnings from boost/concept_check.hpp
Rationalise uses of GCC and clang diagnostic push/pop,
with adding constraint for GCC 4.6+ as minimum version
where the pragma was introduced.
Closes#228