The new warning has two purposes: First, it makes clearer to the
user that it is about OpenMP and, secondly and more importantly,
it permits to use -Wno-openmp.
The newly added -Wopenmp is enabled by default and replaces the
'0' (always warning) in several OpenMP-related warning calls.
For code shared with OpenACC, it only uses OPT_Wopenmp for
'flag_openmp | flag_openmp_simd'.
* lang.opt (Wopenmp): Add, enabled by dafault and documented in C.
* openmp.cc (gfc_match_omp_declare_target, resolve_positive_int_expr,
resolve_nonnegative_int_expr, resolve_omp_clauses,
gfc_resolve_omp_do_blocks): Use OPT_Wopenmp with gfc_warning{,_now}.