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/linuxARM64Gcc/c

@@ -1,3 +1,4 @@
+#------------------------------------------------------------------------------
 include $(GENERAL_RULES)/Gcc/c
 
 cARCH       =
@@ -10,3 +11,5 @@ cFLAGS      = $(cARCH) $(GFLAGS) $(cWARN) $(cOPT) $(cDBUG) $(LIB_HEADER_DIRS) -f
 ctoo        = $(WM_SCHEDULER) $(cc) $(cFLAGS) -c $< -o $@
 
 include $(GENERAL_RULES)/Gcc/link-c
+
+#------------------------------------------------------------------------------