is my postgresql-server thread-safe?
The thread-safety parameter is only relevant for the client library libpq it doesn't influence how the backend is build. The backend actually doesn't even use threads, it uses a process per connection.

edit
just noticed I didn't address the renaming of the configure parameter. The point with configure is that the build scripts can support optional features. If you have a feature X you can pass either --enable-X or --disable-X. However it's help output will only list one of the two based on the current default (enabled is the default since 9.0 for thread-safety).