ENH: extend use of WM_COMPILER_CONTROL to manage compiler versions

- for compilers such as gcc and clang, may have several different
  variants installed on the computer. Use WM_COMPILER_CONTROL to
  specify the preferred variant.

  Eg,
      WM_COMPILER=Gcc
      WM_COMPILER_CONTROL="version=8"

      will compile with "gcc-8" and "g++-8"

  Good practice would be to tag output directory names with the
  version too. Eg

      WM_COMPILER=Clang110
      WM_COMPILER_CONTROL="version=11.0"

STYLE: modify message for change of gcc -> clang (darwin)

wmake/rules/linux64Icc/c++

@@ -8,9 +8,9 @@ include $(DEFAULT_RULES)/c++$(WM_COMPILE_OPTION)
 c++FLAGS    = $(c++ARCH) $(GFLAGS) $(c++WARN) $(c++OPT) $(c++DBUG) $(ptFLAGS) $(LIB_HEADER_DIRS) -fPIC
 
 Ctoo        = $(WM_SCHEDULER) $(CC) $(c++FLAGS) -c $< -o $@
-cxxtoo      = $(Ctoo)
 cctoo       = $(Ctoo)
 cpptoo      = $(Ctoo)
+cxxtoo      = $(Ctoo)
 
 include $(GENERAL_RULES)/Icc/link-c++