cmake NG_HAVE_AVX2 examples

NGSolve/netgen CMakeLists.txt :554

check_cxx_source_runs("
                #include <immintrin.h>
                int main()
                {
                    __m256i a{1,2,3,4};
                    __m256i b{2,0,3,5};
                    __m256i c = _mm256_cmpgt_epi64 (a,b);
                    return 0;
                } " NG_HAVE_AVX2)

NGSolve/netgen CMakeLists.txt :576

elseif(NG_HAVE_AVX2)