Alut's pkg-config definition only adds the top-level include directory
containing the AL directory to the include paths. This works on most
platforms because OpenAL adds that AL directory. However, with nix,
these two directories are distinct and the build fails.
Not only is FMOD neither free (libre) nor free (gratis), the version we
support(ed) is also impossible to legally obtain anymore. So there's no
reason we should keep code around that (pretends to) support a library
nobody can use or test.