CONFIG: handle openmp on Darwin (#1656)

- requires -Xpreprocessor
- uses 'libomp' (no 'libgomp' link)

wmake/rules/General/Clang/openmp

-# Flags for compiling/linking openmp
-# The USE_OMP is for OpenFOAM-specific use (general use is _OPENMP)
-# -
-# Clang provides 'omp' and a link for 'gomp'.
-# With 'gomp' we can use system libs.
-
-COMP_OPENMP = -DUSE_OMP -fopenmp
-LINK_OPENMP = -lgomp

wmake/rules/General/Clang/openmp

+openmp-gomp
\ No newline at end of file

wmake/rules/General/Clang/openmp-gomp

+# Flags for compiling/linking openmp
+# The USE_OMP is for OpenFOAM-specific use (general use is _OPENMP)
+# -
+# Clang provides 'omp' and a link for 'gomp'.
+# With 'gomp' we can also use system libs.
+
+COMP_OPENMP = -DUSE_OMP -fopenmp
+LINK_OPENMP = -lgomp

wmake/rules/General/Clang/openmp-omp

+# Flags for compiling/linking openmp
+# The USE_OMP is for OpenFOAM-specific use (general use is _OPENMP)
+
+COMP_OPENMP = -DUSE_OMP -fopenmp
+LINK_OPENMP = -lomp

wmake/rules/General/mplib

-PFLAGS     =
-PINC       =
-PLIBS      =

wmake/rules/General/mplib

+no-mpi
\ No newline at end of file

wmake/rules/General/no-mpi

+# Empty flags for not compiling/linking MPI
+
+PFLAGS     =
+PINC       =
+PLIBS      =
+
+#------------------------------------------------------------------------------

wmake/rules/General/no-openmp

+# Empty flags for not compiling/linking OPENMP
+
+COMP_OPENMP =
+LINK_OPENMP =
+
+#------------------------------------------------------------------------------

wmake/rules/darwin64Clang/general

@@ -3,7 +3,16 @@ CPP        = cpp -traditional-cpp $(GFLAGS)
 PROJECT_LIBS = -l$(WM_PROJECT) -ldl
 
 include $(GENERAL_RULES)/standard
-include $(GENERAL_RULES)/Clang/openmp
+## include $(GENERAL_RULES)/Clang/openmp
+
+# Darwin-specific
+# ----
+COMP_OPENMP = -DUSE_OMP -Xpreprocessor -fopenmp
+LINK_OPENMP = -lomp
+# ----
+# Or disable
+# include $(GENERAL_RULES)/no-openmp
+# ----
 
 include $(DEFAULT_RULES)/c
 include $(DEFAULT_RULES)/c++