This isn't Madeira proper yet, but something small and useful on it's own, I hope: MADEIRA-PORT.
Main feature is :MADEIRA-PORT ASDF component class:
(defsystem :foo :defsystem-depends-on (:madeira-port) :serial t :components ((:file "package") (:module "ports" :components ((:madeira-port "sbcl" :when :sbcl) (:madeira-port "ccl" :when :ccl) (:madeira-port "ansi" :unless (:or :sbcl :ccl)))) (:file "foo")))
The :WHEN and :UNLESS options support an extended feature syntax, which allows things such as:
(:find-package :swank) (:find-function #:exit :sb-ext)
This extended feature syntax is also accessible by calling EXTEND-FEATURE-SYNTAX at the toplevel, after which regular #+ and #- readmacros will also understand it -- but unfortunately they will also lose any implementation specific extensions in the process.