build-system/haskell: install config for any package that creates it.

A Cabal package is allowed to declare an "empty" library, in an
otherwise executable-only package, for the purpose of allowing Cabal
to use it as a dependency for other packages.  See e.g. hspec-discover.

* guix/build/haskell-build-system.scm (register): Unconditionally call
  setup script with "register", and install any config file generated.

guix/build/haskell-build-system.scm

@@ -166,13 +166,13 @@ (define* (register #:key name system inputs outputs #:allow-other-keys)
                                     (package-name-version haskell)
                                     "/package.conf.d"))
          (id-rx (make-regexp "^id: *(.*)$"))
-         (lib-rx (make-regexp "lib.*\\.(a|so)"))
-         (config-file (string-append config-dir "/" name ".conf"))
+         (config-file (string-append out "/" name ".conf"))
          (params
           (list (string-append "--gen-pkg-config=" config-file))))
-    (unless (null? (find-files lib lib-rx))
+    (run-setuphs "register" params)
+    ;; The conf file is created only when there is a library to register.
+    (when (file-exists? config-file)
       (mkdir-p config-dir)
-      (run-setuphs "register" params)
       (let ((config-file-name+id
              (call-with-ascii-input-file config-file (cut grep id-rx <>))))
         (rename-file config-file