Add FreeBSD support

Compilation failed on FreeBSD, as doesDirectoryExist is imported if `__LINUX__` is defined, but called if neither `__MACOSX__` nor `__WINDOWS__` are defined. The patch fixes this by only calling it if `__LINUX__` is defined (so, if it was imported), uses the correct CA file for FreeBSD (installed by the port `security/ca_root_nss`), and falls back on not verifying the certificates for other OSes. Support for other Unix variants shouldn't be too hard to add too, but I don't have any other right now to test (from what I gathered, OpenBSD stores a `cert.pem` directly in `/etc/ssl`, DragonFlyBSD does the same as FreeBSD, and I don't know about NetBSD nor other Unixes).