Use correct OpenSSL thread safety macros on OS X (closes #209) Create an Autoconf test to determine if pthread_t is a pointer or scalar type, and use the appropriate CRYPTO_THREADID_set macro based on the result.

signature S = sig
        type t
        val x : t
end

signature S' = sig
        include S
        val y : t
end

signature S'' = sig
        type u
        include S' where type t = int
        type v
end