CONFIG: support FOAM_EXTRA_CXXFLAGS (#1256)

- allows custom tuning of compilation parameters

etc/config.csh/unset

@@ -9,8 +9,7 @@
 #     Copyright (C) 2016-2019 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/unset
@@ -46,20 +45,15 @@ endif
 
 unsetenv WM_ARCH
 unsetenv WM_ARCH_OPTION
-unsetenv WM_CC
-unsetenv WM_CFLAGS
 unsetenv WM_COMPILER
 unsetenv WM_COMPILER_ARCH
 unsetenv WM_COMPILER_TYPE
 unsetenv WM_COMPILER_LIB_ARCH
 unsetenv WM_COMPILE_OPTION
-unsetenv WM_CXX
-unsetenv WM_CXXFLAGS
 unsetenv WM_DIR
 unsetenv WM_HOSTS
 unsetenv WM_LABEL_OPTION
 unsetenv WM_LABEL_SIZE
-unsetenv WM_LDFLAGS
 unsetenv WM_MPLIB
 unsetenv WM_NCOMPPROCS
 unsetenv WM_OPTIONS
@@ -75,6 +69,11 @@ unsetenv WM_PROJECT_VERSION
 unsetenv WM_SCHEDULER
 unsetenv WM_THIRD_PARTY_DIR
 
+# Older variables
+
+# Before 1906
+unsetenv WM_CC WM_CXX WM_CFLAGS WM_CXXFLAGS WM_LDFLAGS
+
 
 #------------------------------------------------------------------------------
 # Unset FOAM_* environment variables
@@ -84,8 +83,9 @@ unsetenv FOAM_APPBIN
 unsetenv FOAM_APP
 unsetenv FOAM_CODE_TEMPLATES
 unsetenv FOAM_ETC
+unsetenv FOAM_EXTRA_CXXFLAGS
+unsetenv FOAM_EXTRA_LDFLAGS
 unsetenv FOAM_EXT_LIBBIN
-unsetenv FOAM_INST_DIR
 unsetenv FOAM_JOB_DIR
 unsetenv FOAM_LIBBIN
 unsetenv FOAM_MPI
@@ -102,6 +102,10 @@ unsetenv FOAM_USER_APPBIN
 unsetenv FOAM_USER_LIBBIN
 unsetenv FOAM_UTILITIES
 
+# Old variables
+
+# Before 1812
+unsetenv FOAM_INST_DIR
 
 #------------------------------------------------------------------------------
 # Unset MPI-related environment variables