Adhere to FreeBSD defaults

[scheibel]

In glbinding, @murashin opened a PR added usr/local/include as default include path for FreeBSD (cginternals/glbinding#241). Upon research, I found the usr/local prefix to be the default directory for non-system libraries (source: https://www.freebsd.org/doc/handbook/dirstructure.html). I suggest we adapt this default include path (and probably linker path, too?) for FreeBSD and probably other Unix-based systems.

Furthermore, there may be other FreeBSD default we want to consider.