Should be safe on OS X 10.4 (32-bit), since Apple's OSStatus boils
down to "signed long", and g_set_error() takes gint, which is really
just "int".  Assigning "signed long" to "int" on 32-bit Unix should be
just fine, since both are signed 32-bit ints.

No idea if this is safe on 64-bit OS X.