Port ATTRIBUTE_MAY_ALIAS to recent icc

* src/conf_post.h (ATTRIBUTE_MAY_ALIAS) [__ICC]:
Define to empty.  Otherwise, icc (ICC) 17.0.4 20170411 says
“warning #2621: attribute "__may_alias__" does not apply here”
for constructs like ‘struct sockaddr *sa = (whatever);
struct sockaddr_in __attribute__ ((__may_alias__)) *sin
= (struct sockaddr_in *) sa;’.

src/conf_post.h

@@ -263,7 +263,7 @@ extern int emacs_setenv_TZ (char const *);
 #define ATTRIBUTE_CONST _GL_ATTRIBUTE_CONST
 #define ATTRIBUTE_UNUSED _GL_UNUSED
 
-#if GNUC_PREREQ (3, 3, 0)
+#if GNUC_PREREQ (3, 3, 0) && !defined __ICC
 # define ATTRIBUTE_MAY_ALIAS __attribute__ ((__may_alias__))
 #else
 # define ATTRIBUTE_MAY_ALIAS