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.

libc/runtime/sysconf.c

@@ -51,7 +51,6 @@
  * @raise EINVAL if `name` isn't valid
  */
 long sysconf(int name) {
-  int n;
   switch (name) {
     case _SC_CLK_TCK:
       return CLK_TCK;