Don't rely on otp_release when picking hmac function to use.

Erlang/OTP 17 [erts-6.0] [source] [64-bit] [smp:8:8] [async-threads:10] [hipe] [kernel-poll:false]

Eshell V6.0  (abort with ^G)
1> erlang:system_info(otp_release).
"17"

So the old check would incorrectly use deprecated function on
Erlang/OTP 17. Instead, just check if desired function is exported or
not.

Change-Id: I0896147866c154d183641f571957554e763ba318
Reviewed-on: http://review.couchbase.org/35605
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>