Declare several parameters/variables constant

Add the "const" keyword to parameters and variables that are never
modified. Most of these were spotted by "-Wwrite-strings".

We cast the second parameter to execvp() explicitly as it expects a
"char *const[]" where it should expect a "const char *const[]"
(according to the documentation, this is due to compatibility reasons).
This should be changed once we come up with a better solution.

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

src/notify.c

@@ -119,9 +119,9 @@ notify_bar (void)
 void
 notify_init_vars (void)
 {
-  char *time_format = "%T";
-  char *date_format = "%a %F";
-  char *cmd = "printf '\\a'";
+  const char *time_format = "%T";
+  const char *date_format = "%a %F";
+  const char *cmd = "printf '\\a'";
 
   pthread_mutex_init (&nbar.mutex, NULL);
   nbar.show = 1;