tuntap doesn't support multiqueue on kernel < 3.8

[Jimmy-Z]

According to https://www.kernel.org/doc/Documentation/networking/tuntap.txt

3.3 Multiqueue tuntap interface:

  From version 3.8, Linux supports multiqueue tuntap which ...

I just found out the only thing preventing phantun from working on my oldest server is this, just use try_build instead of try_build_mq.

I could make a pull request if interested, multiqueue will still be on by default but with an option to turn it off.