build: check -Wsubobject-linkage in C++

* configure.ac (warn_cxx): here.

TODO

@@ -1,4 +1,8 @@
 * Soon
+** yyerror_range in GLR
+It seems useless that it is based on glr_stack_item, it should be doable
+with location only.
+
 ** scan-code
 The default case is scanning char-per-char.
 

configure.ac

@@ -137,7 +137,8 @@ if test "$enable_gcc_warnings" = yes; then
   warn_c='-Wbad-function-cast
     -Wmissing-prototypes
     -Wstrict-prototypes'
-  warn_cxx='-Wextra-semi -Wnoexcept -Wold-style-cast -Wundefined-func-template
+  warn_cxx='-Wextra-semi -Wnoexcept -Wsubobject-linkage
+    -Wold-style-cast -Wundefined-func-template
     -Wweak-vtables'
   # Warnings for the test suite only.
   #