Update configuration for BSD target_os

`dragonfly` was moved to `dragonflybsd`, and `bitrig` is no longer a
target. Adjust `cfg!` statements to match this.

src/lib.rs

@@ -837,8 +837,7 @@ impl Config {
                         || cfg!(target_os = "openbsd")
                         || cfg!(target_os = "netbsd")
                         || cfg!(target_os = "freebsd")
-                        || cfg!(target_os = "bitrig")
-                        || cfg!(target_os = "dragonflybsd")) =>
+                        || cfg!(target_os = "dragonfly")) =>
                 {
                     use_jobserver = true;
                     cmd.env("MAKEFLAGS", makeflags);