configure: add -Wno-error=stringop-overflow also to CXXFLAGS

Also c++ programs need -Wno-error=stringop-overflow flag, in order to
avoid errors during linking, when using LTO.

Signed-off-by: Jere Leppänen <jere.leppanen@nokia.com>
Reviewed-by: Janne Peltonen <janne.peltonen@nokia.com>

configure.ac

@@ -120,6 +120,7 @@ AS_IF([test "$GCC" == yes],
       AS_IF([test `$CC -dumpversion | cut -d '.' -f 1` -ge 10],
 	    ODP_CHECK_CFLAG([-Wno-error=array-bounds])
 	    ODP_CHECK_CFLAG([-Wno-error=stringop-overflow])
+	    ODP_CHECK_CXXFLAG([-Wno-error=stringop-overflow])
       )
 )