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/ical.c

@@ -68,7 +68,7 @@ static void ical_export_apoints (FILE *);
 static void ical_export_todo (FILE *);
 static void ical_export_footer (FILE *);
 
-static char *ical_recur_type[RECUR_TYPES] =
+static const char *ical_recur_type[RECUR_TYPES] =
   { "", "DAILY", "WEEKLY", "MONTHLY", "YEARLY" };
 
 /* iCal alarm notification. */