supposedly improved pthread support

* bumped version to 3.7.3
* set pthread thread cancel type to PTHREAD_CANCEL_ASYNCHRONOUS
* lane_h:cancel() accepts a 3rd timeout argument used when waiting for
actual thread termination (hitting the timeout raises an error)
* added PROPAGATE_ALLOCF macro to select state creation mode
(lua_newstate or luaL_newstate)
Fix a macro check