Define _BSD_SOURCE macro for getline on FreeBSD

Define _BSD_SOURCE macro for getline on FreeBSD

According to manpage:

https://www.freebsd.org/cgi/man.cgi?query=getline&apropos=0&sektion=0&manpath=FreeBSD+8.2-RELEASE&format=html

FreeBSD requires either `_WITH_GETLINE`, `_BSD_SOURCE` or `_GNU_SOURCE`
defined to make `getline` available via `<stdio.h>`.

By adding a definition in CMake is the cleanest way as otherwise we get:

> macro name is a reserved identifier

and muting it with `pragma` will be a messier solution
(e.g. dotnet/corefx@ba88726).

Commit migrated from https://github.com/dotnet/corefx/commit/03ac51a99d963f5324eb1b749d3e425d21ec2115