simplify ext/ffi switching logic
simplify ext/ffi switching logic

h4ck3rm1k3/ffi-yajl,h4ck3rm1k3/ffi-yajl,kou/ffi-yajl,chef/ffi-yajl,h4ck3rm1k3/ffi-yajl,kou/ffi-yajl,chef/ffi-yajl,chef/ffi-yajl,kou/ffi-yajl