Need to turn off unsafe buffer warning.

[VictorEijkhout]

rangev3-0.12.0/include/meta/meta.hpp:2277:63: error: 'first'
is an unsafe pointer used for buffer access [-Werror,-Wunsafe-buffer-usage]
        constexpr std::size_t find_index_i_(bool const *const first, bool const *const last,
                                            ~~~~~~~~~~~~~~~~~~^~~~~

My attempt to turn off warning-as-error failed. Cmake option -DCMAKE_COMPILE_WARNING_AS_ERROR=OFF but there is still a -Werror on the compile line.