Fix use of non-portable == in configure.ac.

Fixes bug 34233.

(This has bug has been backported to 0.3.5, but only released in
0.4.3, so it only needs a changes file there.)

configure.ac

@@ -368,7 +368,7 @@ AC_CACHE_CHECK([for __attribute__((fallthrough))],
 	 [tor_cv_c_attr_fallthrough=no] )])
 CFLAGS="$saved_CFLAGS"
 
-if test "$tor_cv_c_attr_fallthrough" == "yes"; then
+if test "$tor_cv_c_attr_fallthrough" = "yes"; then
   AC_DEFINE(HAVE_ATTR_FALLTHROUGH, [1], [defined if we have the fallthrough attribute.])
 fi