CONFIG: add clang third-party lib to LD_LIBRARY_PATH

- required if there is no system openmp and libomp or libgomp are
  only found in the clang hierarchy

STYLE: add some notes in the openmp rules.

- the _OPENMP macro is now used in low-level testing files

applications/test/openmp/Make/options

-EXE_INC = ${COMP_OPENMP}
+EXE_INC = ${COMP_OPENMP} /* -UUSE_OMP */
 
 /* Mostly do not need to explicitly link openmp libraries */
 /* EXE_LIBS = ${LINK_OPENMP} */

applications/test/openmp/Test-openmp.C

@@ -30,7 +30,7 @@ Description
 #include <cstdlib>
 #include <iostream>
 
-#ifdef USE_OMP
+#if _OPENMP
 #include <omp.h>
 #endif
 
@@ -39,11 +39,19 @@ Description
 
 int main(int argc, char *argv[])
 {
+#if USE_OMP
+    std::cout << "USE_OMP defined (" << USE_OMP << ")\n";
+#else
+    std::cout << "USE_OMP undefined\n";
+#endif
+
+#if _OPENMP
+    std::cout << "_OPENMP = " << _OPENMP << "\n\n";
+
+    // Fork threads with their own copies of variables
     int nThreads, threadId;
 
-// Fork threads with their own copies of variables
-#ifdef USE_OMP
-#pragma omp parallel private(nThreads, threadId)
+    #pragma omp parallel private(nThreads, threadId)
     {
         threadId = omp_get_thread_num();
         nThreads = omp_get_num_threads();

etc/config.csh/settings

@@ -302,6 +302,7 @@ CLANG_NOT_FOUND
 
     _foamAddMan     $clangDir/man
     _foamAddPath    $clangDir/bin
+    _foamAddLib     $clangDir/lib  # For things like libomp (openmp) etc
 
     if ($?FOAM_VERBOSE && $?prompt) then
         echo "Using ThirdParty compiler"

etc/config.sh/settings

@@ -287,6 +287,7 @@ CLANG_NOT_FOUND
 
     _foamAddMan     $clangDir/share/man
     _foamAddPath    $clangDir/bin
+    _foamAddLib     $clangDir/lib  # For things like libomp (openmp) etc
 
     if [ "$FOAM_VERBOSE" -a "$PS1" ]
     then

wmake/rules/linux64Clang/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/linux64Cray/openmp

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

wmake/rules/linux64Gcc/openmp

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

wmake/rules/linux64GccKNL/openmp

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

wmake/rules/linux64Icc/openmp

 # Flags for compiling/linking openmp
+# The USE_OMP is for OpenFOAM-specific use (general use is _OPENMP)
 
 COMP_OPENMP = -DUSE_OMP -qopenmp
 LINK_OPENMP = -liomp5

wmake/rules/linux64IccKNL/openmp

 # Flags for compiling/linking openmp
+# The USE_OMP is for OpenFOAM-specific use (general use is _OPENMP)
 
 COMP_OPENMP = -DUSE_OMP -qopenmp
 LINK_OPENMP = -liomp5

wmake/rules/linuxARM64Clang/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/linuxARM64Gcc/openmp

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

wmake/rules/linuxARM7Gcc/openmp

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

wmake/rules/linuxClang/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/linuxGcc/openmp

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

wmake/rules/linuxIA64Gcc/openmp

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

wmake/rules/linuxIA64Icc/openmp

 # Flags for compiling/linking openmp
+# The USE_OMP is for OpenFOAM-specific use (general use is _OPENMP)
 
 COMP_OPENMP = -DUSE_OMP -qopenmp
 LINK_OPENMP = -liomp5

wmake/rules/linuxPPC64Gcc/openmp

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

wmake/rules/linuxPPC64leGcc/openmp

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