To compile this vala code:

    if (value in values)

where value is a string and values a string[], valac generated a function whose prototype is:
    static gboolean _vala_string_array_contains (gchar** stack, int stack_length, gchar* needle);

but the values it passes to the needle params is contant, and hence I got this error:

variable/Options.c: In function 'retro_option_set_current':
variable/Options.c:272:59: warning: passing argument 3 of '_vala_string_array_contains' discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
  if (_vala_string_array_contains (_tmp1_, _tmp1__length1, _tmp0_)) {
                                                           ^
variable/Options.c:253:17: note: expected 'gchar * {aka char *}' but argument is of type 'const gchar * {aka const char *}'
 static gboolean _vala_string_array_contains (gchar** stack, int stack_length, gchar* needle) {
                 ^

I would suggest to make the stack and needle parameters constant as they are never changed by the function.