build: Do not require funopen() to be ported

build: Do not require funopen() to be ported

This function cannot be easily and (more importantly) correctly ported
without cooperation from the libc stdio layer. We already document that
users should be prepared to have the function not available on some
platforms and that they should ideally switch their code to other
more portable and better interfaces.

Instead of making the build fail, and requiring porters to add
exceptions for something that most probably cannot be ported correctly
anyway, simply print a warning and let it build. This will not be a
regression because on those systems libbsd would have never been built
before.

Prompted-by: Jens Finkhaeuser <jens@finkhaeuser.de>