Use an unsigned char for bool if we don't use the native bool.

On (rare) platforms where sizeof(bool) > 1, we need to use our own
bool, but imported c99 code (such as Ryu) may want to use bool values
as array subscripts, which elicits warnings if bool is defined as
char. Using unsigned char instead should work just as well for our
purposes, and avoid such warnings.

Per buildfarm members prariedog and locust.

src/include/c.h

@@ -305,7 +305,7 @@
 #else
 
 #ifndef bool
-typedef char bool;
+typedef unsigned char bool;
 #endif
 
 #ifndef true