Cast the input to isalpha et al to integer

* libguile/gc_os_dep.c (GC_linux_stack_base) [LINUX_STACKBOTTOM]: cast
  input of ctype functions to int

* libguile/inet_aton.c (inet_aton): cast input of ctype functions to int

* libguile/read.c (scm_scan_for_encoding): cast input of isalnum to int

* libguile/win32-socket.c (scm_i_socket_uncomment): cast input of isspace
  to int

libguile/win32-socket.c

@@ -435,7 +435,7 @@ scm_i_socket_uncomment (char *line)
       while (end > line && (*end == '\r' || *end == '\n'))
 	*end-- = '\0';
     }
-  while (end > line && isspace (*end))
+  while (end > line && isspace ((int) (*end)))
     *end-- = '\0';
 
   return end;