[simd] Make compiler ignore (no warning) unknown pragmas

This suppressed the warning for the opm pragma, and all other such warnings.

assigned to @christi

Looks good.

merged

mentioned in commit 9d226586

This doesn't seem to work for some reason.

Seem to be a known bug in gcc which hasn't been fixed in almost 10 years https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53431

It does not work because #pragmas are preprocessor directives and the preprocessor in c++ ignores the "diagnostic ignored" statements to disable the warnings.

well it works for clang but not for gcc and is a reported bug for gcc, so at least some people agree that this is not the desired behaviour.

But it looks like a bug that will not be fixed. So, we have to find an alternative solution, because these warnings are really annoying.

mentioned in issue #267 (closed)

I suggest to introduce a conditional #pragma, e.g., something like

#if _OPENMP >= 201307
  #define DUNE_PRAGMA_OMP_SIMD _Pragma("omp simd")
#else 
  #define DUNE_PRAGMA_OMP_SIMD
#endif

sounds good

See !1035 (merged)

mentioned in merge request !1035 (merged)