Fix checks for openpty() on FreeBSD

[b4n]

FreeBSD has openpty() in libutil.h.

I PR this because while the Waf changes work, they might be deemed ugly. @eht16 if you have a better version for this it'd be welcome :) Otherwise they are OK enough with me.

[techee]

Just tested on OS X - works fine here.

[eht16]

Since both kwargs are passed explicitly to check_cc() below, we probably should .pop() them from the dict to not pass them twice to check_cc()

[eht16]

Except the both code comments this looks fine to me. Why did you think it would be that ugly? 😄

[b4n]

@eht16

I updated the commit to address your remarks. Looks way better now indeed :)

Why did you think it would be that ugly?

Well, a combination of what your remarks fixed, and that I don't find it particularly clean to try…except in the loop just to re-raise the exception if the loop exited. But I couldn't really find another way to do this, and it works, so it might be good enough ^^

[eht16]

I agree on the part that re-raising a remembered exception is not perfectly nice. Still it is ok I guess.
If anyone comes up with a cleaner solution later, we can improve it then as well.