Ensure that <stdint.h> defines SIZE_MAX and friends.

We were missing SIG_ATOMIC_MAX, SIG_ATOMIC_MIN, SIZE_MAX,
WCHAR_MAX, WCHAR_MIN, WINT_MAX, and WINT_MIN.

Change-Id: I2535f36bc220fbaea009b483599b7af811c4cb5c
diff --git a/libc/include/wchar.h b/libc/include/wchar.h
index a4e19f0..76ac02c 100644
--- a/libc/include/wchar.h
+++ b/libc/include/wchar.h
@@ -65,8 +65,11 @@
     WC_TYPE_MAX
 } wctype_t;
 
+#ifndef WCHAR_MAX
 #define  WCHAR_MAX   INT_MAX
 #define  WCHAR_MIN   INT_MIN
+#endif
+
 #define  WEOF        ((wint_t)(-1))
 
 extern wint_t            btowc(int);