fixup! Only define FALLTHROUGH when __attribute__((fallthrough)) is d…

…etected.

Add -Werror to make sure that we detect it.

configure.ac

@@ -356,6 +356,8 @@ if test "$tor_cv_c_c99_designated_init" != "yes"; then
   AC_MSG_ERROR([Your compiler doesn't support c99 designated initializers. This is required as of Tor 0.2.6.x])
 fi
 
+saved_CFLAGS="$CFLAGS"
+CFLAGS="$CFLAGS -Werror"
 AC_CACHE_CHECK([for __attribute__((fallthrough))],
       tor_cv_c_attr_fallthrough,
       [AC_COMPILE_IFELSE(
@@ -364,6 +366,7 @@ AC_CACHE_CHECK([for __attribute__((fallthrough))],
                                case 2: fn(); break; } ]])],
 	 [tor_cv_c_attr_fallthrough=yes],
 	 [tor_cv_c_attr_fallthrough=no] )])
+CFLAGS="$saved_CFLAGS"
 
 if test "$tor_cv_c_attr_fallthrough" == "yes"; then
   AC_DEFINE(HAVE_ATTR_FALLTHROUGH, [1], [defined if we have the fallthrough attribute.])