gnu: flex: Use Bison 2.7 for tests.

* gnu/packages/bison.scm (bison): Move FLEX to 'inputs', and remove the
  hack that turned off its test suite.
* gnu/packages/flex.scm (flex): Add 'bison-for-tests' in 'inputs', and
  use that to run the tests.

gnu/packages/bison.scm

@@ -40,17 +40,8 @@ (define bison
        (base32
         "1j14fqgi9wzqgsy4fhkcdrv4hv6rrvhvn84axs520w9b022mbb79"))))
     (build-system gnu-build-system)
-    (native-inputs `(("perl" ,perl)
-
-                     ;; We need Flex for the test suite, and Flex needs Bison.
-                     ;; To break the cycle, we remove Bison from the inputs of
-                     ;; Flex, and disable Flex's test suite, since it requires
-                     ;; Bison.
-                     ("flex" ,(package (inherit flex)
-                                (arguments '(#:tests? #f))
-                                (inputs
-                                 (alist-delete "bison"
-                                               (package-inputs flex)))))))
+    (native-inputs `(("perl" ,perl)))
+    (inputs `(("flex" ,flex)))
     (propagated-inputs `(("m4" ,m4)))
     (home-page "http://www.gnu.org/software/bison/")
     (synopsis "Parser generator")

gnu/packages/flex.scm

@@ -25,6 +25,7 @@ (define-module (gnu packages flex)
   #:use-module (gnu packages m4)
   #:use-module (gnu packages bison)
   #:use-module (gnu packages indent)
+  #:use-module (srfi srfi-1)
   #:export (flex))
 
 (define flex
@@ -41,9 +42,26 @@ (define flex
     (build-system gnu-build-system)
     (arguments
      '(#:patches (list (assoc-ref %build-inputs "patch/bison-tests"))))
-    (inputs `(("patch/bison-tests" ,(search-patch "flex-bison-tests.patch"))
-              ("bison" ,bison)
-              ("indent" ,indent)))
+    (inputs
+     (let ((bison-for-tests
+            ;; Work around an incompatibility with Bison 3.0:
+            ;; <http://lists.gnu.org/archive/html/bug-bison/2013-09/msg00014.html>.
+            (package (inherit bison)
+              (version "2.7.1")
+              (source (origin
+                       (method url-fetch)
+                       (uri (string-append "mirror://gnu/bison/bison-"
+                                           version ".tar.xz"))
+                       (sha256
+                        (base32
+                         "1yx7isx67sdmyijvihgyra1f59fwdz7sqriginvavfj5yb5ss2dl"))))
+
+              ;; Unlike Bison 3.0, this version did not need Flex for its
+              ;; tests, so it allows us to break the cycle.
+              (inputs (alist-delete "flex" (package-inputs bison))))))
+       `(("patch/bison-tests" ,(search-patch "flex-bison-tests.patch"))
+         ("bison" ,bison-for-tests)
+         ("indent" ,indent))))
     (propagated-inputs `(("m4" ,m4)))
     (home-page "http://flex.sourceforge.net/")
     (synopsis "A fast lexical analyser generator")