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/tinymath/feval.internal.h

@@ -5,14 +5,17 @@ COSMOPOLITAN_C_START_
 
 static inline void fevalf(float x) {
   volatile float y = x;
+  (void)y;
 }
 
 static inline void feval(double x) {
   volatile double y = x;
+  (void)y;
 }
 
 static inline void fevall(long double x) {
   volatile long double y = x;
+  (void)y;
 }
 
 COSMOPOLITAN_C_END_