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/linuxARM64Arm/c

@@ -1,7 +1,9 @@
-# A clang variant
+#------------------------------------------------------------------------------
+# ARM compiler - a clang variant
+#------------------------------------------------------------------------------
 include $(GENERAL_RULES)/Clang/c
 
-cc          = armclang
+cc          = armclang$(COMPILER_VERSION)
 
 cARCH       = -mcpu=native
 
@@ -16,3 +18,5 @@ include $(GENERAL_RULES)/Clang/link-c
 
 LINKLIBSO  += -armpl
 LINKEXE    += -armpl
+
+#------------------------------------------------------------------------------