build: Add -Werror=implicit-fallthrough compile flag

configure.ac

@@ -433,6 +433,7 @@ if test "x$enable_werror" = "xyes"; then
                         [AC_LANG_SOURCE([[struct A { virtual void f(); }; struct B : A { void f() final; };]])])
   AX_CHECK_COMPILE_FLAG([-Werror=unreachable-code-loop-increment],[ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=unreachable-code-loop-increment"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Werror=mismatched-tags], [ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=mismatched-tags"], [], [$CXXFLAG_WERROR])
+  AX_CHECK_COMPILE_FLAG([-Werror=implicit-fallthrough], [ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=implicit-fallthrough"], [], [$CXXFLAG_WERROR])
 
   if test x$suppress_external_warnings != xno ; then
     AX_CHECK_COMPILE_FLAG([-Werror=documentation],[ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=documentation"],,[[$CXXFLAG_WERROR]])