[CMake] Pass -lpthread on FreeBSD.
[CMake] Pass -lpthread on FreeBSD.

At the time this code was originally written, Swift standard
library hasn't been ported to FreeBSD, so we didn't need any
additional library. This is not true anymore, so fix it.

khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift