Support cross compiling

[soxrok2212]

This is a little bit of a hack, but it allows for cross compiling out-of-the-box. There are probably better ways about this, but this is what I came up with that worked when specifying custom libpcap and libnet paths.

Caveats:

Tests are disabled when libpcap/libnet is specified Feedback is appreciated. Tested on arm64: native arm64 binary builds and mips64 cross compile works.