gnu: pies: Remove pre-generated files.

* gnu/packages/admin.scm (pies)[source]: Add snippet to remove
pre-generated files.
[native-inputs]: Add bison, flex.

gnu/packages/admin.scm

@@ -792,7 +792,16 @@ (define-public pies
                            version ".tar.bz2"))
        (sha256
         (base32
-         "0v0xcq0mfil440xq2pa5mjkyva5c9ahqda54z5w2ksl2d78v8a35"))))
+         "0v0xcq0mfil440xq2pa5mjkyva5c9ahqda54z5w2ksl2d78v8a35"))
+       (snippet
+        #~(begin
+            (use-modules (guix build utils))
+            (for-each delete-file
+                      (append
+                        ;; Generated by flex.
+                        (find-files "gres/src" "lex\\.c$")
+                        ;; Generated by bison.
+                        (find-files "gres/src" "-gram\\.[ch]$")))))))
     (build-system gnu-build-system)
     (arguments
      '(#:phases (modify-phases %standard-phases
@@ -804,6 +813,7 @@ (define-public pies
                         (substitute* '("src/progman.c" "src/comp.c")
                           (("\"/bin/sh\"")
                            (string-append "\"" bash "/bin/sh\"")))))))))
+    (native-inputs (list bison flex))
     (home-page "https://www.gnu.org.ua/software/pies/")
     (synopsis "Program invocation and execution supervisor")
     (description