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/bashrc

@@ -6,7 +6,7 @@
 #    \\/     M anipulation  |
 #------------------------------------------------------------------------------
 #     Copyright (C) 2011-2016 OpenFOAM Foundation
-#     Copyright (C) 2016-2020 OpenCFD Ltd.
+#     Copyright (C) 2016-2021 OpenCFD Ltd.
 #------------------------------------------------------------------------------
 # License
 #     This file is part of OpenFOAM, distributed under GPL-3.0-or-later.
@@ -87,6 +87,7 @@ export WM_COMPILE_OPTION=Opt
 # [WM_COMPILE_CONTROL] - additional control for compiler rules
 #   +gold   : with gold linker
 #   ~openmp : without openmp
+#   version=... : compiler suffix (eg, "11" for gcc-11)
 #export WM_COMPILE_CONTROL="+gold"
 
 # [WM_MPLIB] - MPI implementation:

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

etc/config.sh/settings

@@ -78,11 +78,15 @@ Linux)
     esac
     ;;
 
-# Presume x86_64, with clang (not gcc) as system compiler
+# arm64 or x86_64 architectures
 Darwin)
     WM_ARCH=darwin64
-    [ "$WM_COMPILER" = Gcc ] && WM_COMPILER=Clang
-    echo "openfoam: darwin support is clang/llvm only" 1>&2
+    # Defult to clang (not gcc) as system compiler
+    if [ "$WM_COMPILER" = Gcc ]
+    then
+        WM_COMPILER=Clang
+        echo "openfoam (darwin): using clang instead of gcc" 1>&2
+    fi
     ;;
 
 # Presume x86_64, with mingw cross-compiled

etc/cshrc

@@ -87,6 +87,7 @@ setenv WM_COMPILE_OPTION Opt
 # [WM_COMPILE_CONTROL] - additional control for compiler rules
 #   +gold   : with gold linker
 #   ~openmp : without openmp
+#   version=... : compiler suffix (eg, "11" for gcc-11)
 #setenv WM_COMPILE_CONTROL "+gold"
 
 # [WM_MPLIB] - MPI implementation: