Fix FreeBSD compilation (#484)
Fix FreeBSD compilation (#484)

* Fix FreeBSD compilation

On my local test setup I needed more LDFLAGS (linking flags) for the
FreeBSD build to work.

Without it the following error would be returned on Nif.start:

```
warning: Error loading NIF (Is your operating system
  (amd64-portbld-freebsd11.2) supported? Please check
  http://docs.appsignal.com/support/operating-systems.html):
Failed to load NIF library:
  '/path/_build/dev/lib/appsignal/priv/appsignal_extension.so:
    Undefined symbol "appsignal_running_in_container"'
    (appsignal) lib/appsignal/nif.ex:48: Appsignal.Nif.init/0
    (kernel) code_server.erl:1347: anonymous fn/1 in
      :code_server.handle_on_load/5
```

* Fix FreeBSD fatal warnings linking flag (#488)

The linker on FreeBSD 12 and up does not support `-fatal_warnings`
option anymore. Instead use `--fatal-warnings` which does the same
thing.

As reported in #483

Co-authored-by: Kirill Radzikhovskyy <8d7178b88d6e2474f65f633120f286dc7fc0ca4f@gmail.com>

appsignal/appsignal-elixir,appsignal/appsignal-elixir,appsignal/appsignal-elixir,appsignal/appsignal-elixir