Fix build errors on FreeBSD

Hi, I fixed the build errors on FreeBSD. Although I couldn't eliminate all warnings and the CMake files are not perfect, this should be a (big) step forward for BSD support.

Please tell me if you don't like something then I will correct it and modify the PR.

`-DENABLE_DEVELOPMENT` does not work because it treats all warnings as errors..

Related issue: https://github.com/zmap/zmap/issues/110

Best regards,
Clemens Gruber

@hukl: #FS124 :+1: