gnu: flex: Add m4 as a native input.

* gnu/packages/flex (flex): Add m4 as a native input.

gnu/packages/flex.scm

@@ -60,6 +60,8 @@ (define flex
               (inputs (alist-delete "flex" (package-inputs bison))))))
        `(("bison" ,bison-for-tests)
          ("indent" ,indent))))
+    ;; m4 is not present in PATH when cross-building
+    (native-inputs ("m4" ,m4))
     (propagated-inputs `(("m4" ,m4)))
     (home-page "http://flex.sourceforge.net/")
     (synopsis "Fast lexical analyser generator")