CONFIG: provide separate default settings for clang/gcc (fixes #1566)

- the foamConfigurePaths script is quite simplistic and aggressive in
  what it changes. This was particularly evident when using it to
  change gcc/clang versions.

  Restructured the corresponding compiler settings to define default
  versions (eg, "default_gcc_version") that limits the scope of
  changes performed by foamConfigurePaths and makes it easier to
  understand if changing manually.

etc/config.sh/example/compiler

@@ -9,15 +9,17 @@
 #     Copyright (C) 2017 OpenCFD Ltd.
 #------------------------------------------------------------------------------
 # License
-#     This file is part of OpenFOAM, licensed under GNU General Public License
-#     <http://www.gnu.org/licenses/>.
+#     This file is part of OpenFOAM, distributed under GPL-3.0-or-later.
 #
 # File
 #     config.sh/example/compiler
 #     - sourced by OpenFOAM-*/etc/config.sh/settings
 #
 # Description
-#     Example of fine tuning compiler versions and settings for OpenFOAM
+#     Older example of fine tuning compiler settings for OpenFOAM.
+#
+#     The preferred mechanism is now with a "compiler-$WM_COMPILER" file
+#     in one of the etc/ directories.
 #
 #------------------------------------------------------------------------------
 
@@ -26,18 +28,12 @@ eval $($WM_PROJECT_DIR/bin/foamEtcFile -sh -config -mode=o compiler)
 
 # Modify/override compiler settings
 case "$WM_COMPILER" in
-Gcc70)
+Gcc70*)
     gcc_version=gcc-7.0.0
     gmp_version=gmp-6.1.2
     mpfr_version=mpfr-3.1.5
     mpc_version=mpc-1.0.3
     ;;
-Gcc48u)
-    # Example of using the system GCC 4.8 in Ubuntu 15.10. Keep in mind you
-    # will also need to create the respective directory in "wmake/rules"
-    export CC='gcc-4.8'
-    export CXX='g++-4.8'
-    ;;
 esac
 
 #------------------------------------------------------------------------------

etc/config.sh/example/compiler-Gcc92

@@ -0,0 +1,28 @@
+#----------------------------------*-sh-*--------------------------------------
+# =========                 |
+# \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
+#  \\    /   O peration     |
+#   \\  /    A nd           | www.openfoam.com
+#    \\/     M anipulation  |
+#------------------------------------------------------------------------------
+#     Copyright (C) 2020 OpenCFD Ltd.
+#------------------------------------------------------------------------------
+# License
+#     This file is part of OpenFOAM, distributed under GPL-3.0-or-later.
+#
+# File
+#     config.sh/example/compiler-Gcc92
+#     - sourced by OpenFOAM-*/etc/config.sh/compiler
+#
+# Description
+#     Example of fine tuning compiler settings with a hook
+#
+#------------------------------------------------------------------------------
+
+# Modify/override compiler settings
+gcc_version=gcc-9.2.0
+gmp_version=gmp-6.2.0
+mpfr_version=mpfr-4.0.2
+mpc_version=mpc-1.1.0
+
+#------------------------------------------------------------------------------