gnu: lxsession: Build Vala code from source.

* gnu/packages/lxde.scm (lxsession)[source](snippet): New field.

gnu/packages/lxde.scm

@@ -333,7 +333,20 @@ (define-public lxsession
        (patches (search-patches "lxsession-use-gapplication.patch"))
        (sha256
         (base32
-         "1a0zmyywwzdh59nc0l94cir18vhp633z4q2xfhn5zx11ajj45gwh"))))
+         "1a0zmyywwzdh59nc0l94cir18vhp633z4q2xfhn5zx11ajj45gwh"))
+       (modules '((guix build utils)))
+       (snippet
+        ;; Remove C files generated by Vala so we can build from source.
+        '(let* ((c->vala
+                 (lambda (file)
+                   (string-append (string-drop-right file 2)
+                                  ".vala")))
+                (generated-c-file?
+                 (lambda (file stat)
+                   (and (string-suffix? ".c" file)
+                        (file-exists? (c->vala file))))))
+           (for-each delete-file
+                     (find-files "." generated-c-file?))))))
     (build-system gnu-build-system)
     (arguments
      `(#:phases