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.csh/compiler

@@ -6,11 +6,10 @@
 #    \\/     M anipulation  |
 #------------------------------------------------------------------------------
 #     Copyright (C) 2016 OpenFOAM Foundation
-#     Copyright (C) 2016-2019 OpenCFD Ltd.
+#     Copyright (C) 2016-2020 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
 #     etc/config.csh/compiler
@@ -24,14 +23,19 @@
 
 switch ("$WM_COMPILER_TYPE")
 case ThirdParty:
-    # Default versions of GMP, MPFR, MPC - override as necessary
+    # Default versions (CLANG, GCC, GMP, MPFR, MPC) - override as necessary
 
-    set gmp_version=gmp-system
-    set mpfr_version=mpfr-system
-    set mpc_version=mpc-system
+    set default_clang_version=llvm-3.7.1
+    set default_gcc_version=gcc-4.8.5
+
+    set default_gmp_version=gmp-system
+    set default_mpfr_version=mpfr-system
+    set default_mpc_version=mpc-system
 
     switch ("$WM_COMPILER")
     case Gcc:
+        set gcc_version="$default_gcc_version"
+        breaksw
     case Gcc48*:
         set gcc_version=gcc-4.8.5
         breaksw
@@ -95,7 +99,10 @@ case ThirdParty:
     case Gcc92*:
         set gcc_version=gcc-9.2.0
         breaksw
+
     case Clang:
+        set clang_version="$default_clang_version"
+        breaksw
     case Clang37*:
         set clang_version=llvm-3.7.1
         breaksw
@@ -126,6 +133,7 @@ case ThirdParty:
     case Clang90*:
         set clang_version=llvm-9.0.0
         breaksw
+
     default:
         /bin/cat << UNKNOWN_COMPILER
 ===============================================================================
@@ -143,4 +151,7 @@ UNKNOWN_COMPILER
     breaksw
 endsw
 
+unset default_gcc_version default_clang_version
+unset default_gmp_version default_mpfr_version default_mpc_version
+
 #------------------------------------------------------------------------------

etc/config.csh/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.csh/example/compiler
 #     - sourced by OpenFOAM-*/etc/config.csh/settings
 #
 # Description
-#     Example of fine tuning ThirdParty compiler 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.
 #
 #------------------------------------------------------------------------------
 
@@ -26,7 +28,7 @@ eval `$WM_PROJECT_DIR/bin/foamEtcFile -csh -config -mode=o compiler`
 
 # Modify/override compiler settings
 switch ("$WM_COMPILER")
-case Gcc70:
+case Gcc70*:
     set gcc_version=gcc-7.0.0
     set gmp_version=gmp-6.1.2
     set mpfr_version=mpfr-3.1.5

etc/config.csh/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.csh/example/compiler-Gcc92
+#     - sourced by OpenFOAM-*/etc/config.csh/compiler
+#
+# Description
+#     Example of fine tuning compiler settings with a hook
+#
+#------------------------------------------------------------------------------
+
+# Modify/override compiler settings
+set gcc_version=gcc-9.2.0
+set gmp_version=gmp-6.2.0
+set mpfr_version=mpfr-4.0.2
+set mpc_version=mpc-1.1.0
+
+#------------------------------------------------------------------------------