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)

etc/config.csh/settings

@@ -82,11 +82,13 @@ case Linux:
     endsw
     breaksw
 
-# Presume x86_64, with clang (not gcc) as system compiler
+# arm64 or x86_64 architectures
 case Darwin:
     setenv WM_ARCH darwin64
-    if ( "$WM_COMPILER" == Gcc ) setenv WM_COMPILER Clang
-    echo "openfoam: darwin support is clang/llvm only"
+    if ( "$WM_COMPILER" == Gcc ) then
+        setenv WM_COMPILER Clang
+        echo "openfoam (darwin): using clang instead of gcc"
+    endif
     breaksw
 
 # Presume x86_64, with mingw cross-compiled