Fix warnings

This change fixes Cosmopolitan so it has fewer opinions about compiler
warnings. The whole repository had to be cleaned up to be buildable in
-Werror -Wall mode. This lets us benefit from things like strict const
checking. Some actual bugs might have been caught too.

third_party/quickjs/bigint.c

@@ -58,6 +58,7 @@ static JSValue JS_CompactBigInt1(JSContext *ctx, JSValue val,
         return JS_NewInt64(ctx, v);
     } else if (a->expn == BF_EXP_ZERO && a->sign) {
         JSBigFloat *p = JS_VALUE_GET_PTR(val);
+        (void)p;
         assert(p->header.ref_count == 1);
         a->sign = 0;
     }