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

wmake/rules/linux64Cray/openmp

@@ -1,4 +1,5 @@
 # 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