Remove path and version from Racket libc FFI definer
Remove path and version from Racket libc FFI definer

According to [1], Racket recommends specifying `#f` as "library path"
when the goal is to access functions of the C runtime.

This enables using the Racket backend on systems where libc is not
always version "6" (e.g. on FreeBSD it's "7").

[1]: https://docs.racket-lang.org/foreign/Loading_Foreign_Libraries.html#%28def._%28%28lib._ffi%2Funsafe..rkt%29._ffi-lib%29%29

mmhelloworld/idris-jvm,mmhelloworld/idris-jvm,mmhelloworld/idris-jvm,mmhelloworld/idris-jvm,mmhelloworld/idris-jvm,mmhelloworld/idris-jvm