Define the type of marking flags as unsigned int.

We could define it as _Bool since we are going to use C99, but unsigned
int is more portable (even in C89!) and extensible (when we decide to
use tri-color marking GC.)

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>
Merge pull request #23 from omasanori/iso-c-mark

Define the type of marking flags as unsigned int.