ENH: add whichMpicxx function for C++ programs

etc/tools/ThirdPartyFunctions

@@ -98,6 +98,24 @@ whichMpicc()
     echo "${mpicc:-mpicc}"
 }
 
+
+# Return <mpicc> by default or <mpiicc> if possible for INTELMPI.
+# Cray doesn't have <mpicc>, but its <cc> manages mpi paths directly.
+# NOTE: could further refine based on $CC or $WM_CC, but not yet needed
+whichMpicxx()
+{
+    local mpicxx=$(command -v mpicxx)
+    case "$WM_MPLIB" in
+    (INTELMPI)
+        mpicxx=$(command -v mpiicpc)    # Intel <mpiicpc> available?
+        ;;
+    (CRAY-MPI*)
+        : ${mpicxx:=CC}                 # Cray <CC> if there is no <mpicc>
+        ;;
+    esac
+    echo "${mpicxx:-mpicxx}"
+}
+
 # The presence of wmkdep etc required for building with wmake
 requireWMakeToolchain()
 {