CONFIG: support FOAM_EXTRA_CXXFLAGS (#1256)

- allows custom tuning of compilation parameters

etc/config.sh/unset

@@ -6,11 +6,10 @@
 #    \\/     M anipulation  |
 #------------------------------------------------------------------------------
 #     Copyright (C) 2011-2016 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.sh/unset
@@ -35,19 +34,14 @@ foamOldDirs="$WM_PROJECT_DIR $WM_THIRD_PARTY_DIR \
 
 unset WM_ARCH
 unset WM_ARCH_OPTION
-unset WM_CC
-unset WM_CFLAGS
 unset WM_COMPILER
 unset WM_COMPILER_TYPE
 unset WM_COMPILER_LIB_ARCH
 unset WM_COMPILE_OPTION
-unset WM_CXX
-unset WM_CXXFLAGS
 unset WM_DIR
 unset WM_HOSTS
 unset WM_LABEL_OPTION
 unset WM_LABEL_SIZE
-unset WM_LDFLAGS
 unset WM_MPLIB
 unset WM_NCOMPPROCS
 unset WM_OPTIONS
@@ -63,6 +57,10 @@ unset WM_PROJECT_VERSION
 unset WM_SCHEDULER
 unset WM_THIRD_PARTY_DIR
 
+# Older variables
+
+# Before 1906
+unset WM_CC WM_CXX WM_CFLAGS WM_CXXFLAGS WM_LDFLAGS
 
 #------------------------------------------------------------------------------
 # Unset FOAM_* environment variables
@@ -72,8 +70,9 @@ unset FOAM_APPBIN
 unset FOAM_APP
 unset FOAM_CODE_TEMPLATES
 unset FOAM_ETC
+unset FOAM_EXTRA_CXXFLAGS
+unset FOAM_EXTRA_LDFLAGS
 unset FOAM_EXT_LIBBIN
-unset FOAM_INST_DIR
 unset FOAM_JOB_DIR
 unset FOAM_LIBBIN
 unset FOAM_MPI
@@ -90,6 +89,10 @@ unset FOAM_USER_APPBIN
 unset FOAM_USER_LIBBIN
 unset FOAM_UTILITIES
 
+# Old variables
+
+# Before 1812
+unset FOAM_INST_DIR
 
 #------------------------------------------------------------------------------
 # Unset MPI-related environment variables