Fix -Wcast-qual compiler warning

String literals have the type 'char[N]' by default. Allthough they are
not really 'const', modifying a string literal is undefined behaviour.
Therefore, to avoid mistakes, libdivecomputer uses the -Wwrite-strings
compiler option to change the default type to 'const char[N]'.

The cast that triggers the -Wcast-qual warning can be avoided by using a
character array instead of string literal.

examples/dctool.c

@@ -240,7 +240,8 @@ main (int argc, char *argv[])
 	}
 
 	// Translate the help option into a command.
-	char *argv_help[] = {(char *) "help", NULL, NULL};
+	char helpcmd[] = "help";
+	char *argv_help[] = {helpcmd, NULL, NULL};
 	if (help || argv[0] == NULL) {
 		if (argv[0]) {
 			argv_help[1] = argv[0];