Enable -Wextra for the CI builds

Try detecting more warnings that can be useful when building the
library.

Don't use -Wextra with gcc 4.8 which generates several completely
spurious warnings not generated by later compiler versions and which are
not worth fixing/working around.

.github/workflows/ci.yml

@@ -93,6 +93,7 @@ jobs:
             compiler: g++-4.8
             container: ubuntu:18.04
             configure_flags: --disable-shared
+            allow_extra_warnings: true
             use_xvfb: true
           - name: Ubuntu 18.04 wxGTK 3 compatible 3.0
             runner: ubuntu-latest
@@ -233,7 +234,10 @@ jobs:
           esac
 
           if [ -z ${{ matrix.allow_warnings }} ]; then
-            error_opts="-Werror $allow_warn_opt"
+            if [ -z ${{ matrix.allow_extra_warnings }} ]; then
+              error_opts="-Wextra"
+            fi
+            error_opts="$error_opts -Werror $allow_warn_opt"
             echo "wxMAKEFILE_ERROR_CXXFLAGS=$error_opts" >> $GITHUB_ENV
             echo "wxMAKEFILE_CXXFLAGS=$wxMAKEFILE_CXXFLAGS $error_opts" >> $GITHUB_ENV
           fi