Port VECTOR_ELTS_MAX to older GCC etc.

Problem reported by Eli Zaretskii in:
https://lists.gnu.org/r/emacs-devel/2018-12/msg00182.html
* src/alloc.c (VECTOR_ELTS_MAX): Now a macro.
Needed for portability to older GCC and non-GCC.

src/alloc.c

@@ -3336,10 +3336,13 @@ sweep_vectors (void)
     }
 }
 
-static ptrdiff_t const VECTOR_ELTS_MAX
-  = min (((min (PTRDIFF_MAX, SIZE_MAX) - header_size - large_vector_offset)
-	  / word_size),
-	 MOST_POSITIVE_FIXNUM);
+/* Maximum number of elements in a vector.  This is a macro so that it
+   can be used in an integer constant expression.  */
+
+#define VECTOR_ELTS_MAX \
+  min (((min (PTRDIFF_MAX, SIZE_MAX) - header_size - large_vector_offset) \
+	/ word_size), \
+       MOST_POSITIVE_FIXNUM)
 
 /* Value is a pointer to a newly allocated Lisp_Vector structure
    with room for LEN Lisp_Objects.  LEN must be positive and