Add infrastructure for openmp4 compat

src/omp_compat.h

@@ -0,0 +1,35 @@
+/* -*- c++ -*- ----------------------------------------------------------
+   LAMMPS - Large-scale Atomic/Molecular Massively Parallel Simulator
+   http://lammps.sandia.gov, Sandia National Laboratories
+   Steve Plimpton, sjplimp@sandia.gov
+
+   Copyright (2020) Sandia Corporation.  Under the terms of Contract
+   DE-AC04-94AL85000 with Sandia Corporation, the U.S. Government retains
+   certain rights in this software.  This software is distributed under
+   the GNU General Public License.
+
+   See the README file in the top-level LAMMPS directory.
+------------------------------------------------------------------------- */
+
+// There is no way to annotate an OpenMP construct that
+// (a) accesses const variables, (b) has default(none),
+// and (c) is valid in both OpenMP 3.0 and 4.0.
+//
+// (in OpenMP 3.0, the const variables have a predetermined
+//  sharing attribute and are *forbidden* from being declared
+//  in the omp construct.  In OpenMP 4.0, this predetermined
+//  sharing attribute is removed, and thus they are *required*
+//  to be declared in the omp construct)
+//
+// To date, most compilers still accept the OpenMP 3.0 form,
+// so this is what LAMMPS primarily uses.  For those compilers
+// that strictly implement OpenMP 4.0 (such as GCC 9.0), we
+// give up default(none).
+#if LMP_OMP_COMPAT == 4
+#    define LMP_SHARED(...)
+#    define LMP_DEFAULT_NONE default(shared)
+#else
+#    define LMP_SHARED(...) shared(__VA_ARGS__)
+#    define LMP_DEFAULT_NONE default(none)
+#endif
+