flex: bring back 2.6.1 as an option

Some breakages after flex update aren't so easy to fix,
and I can't really afford to spend much time on it ATM.

pkgs/development/tools/parsing/flex/2.6.1.nix

@@ -0,0 +1,37 @@
+{ stdenv, fetchurl, bison, m4 }:
+
+stdenv.mkDerivation rec {
+  name = "flex-2.6.1";
+
+  src = fetchurl {
+    url = "https://github.com/westes/flex/releases/download/v2.6.1/flex-2.6.1.tar.gz";
+    sha256 = "0fy14c35yz2m1n1m4f02by3501fn0cca37zn7jp8lpp4b3kgjhrw";
+  };
+
+  buildInputs = [ bison ];
+
+  propagatedNativeBuildInputs = [ m4 ];
+
+  postConfigure = stdenv.lib.optionalString (stdenv.isDarwin || stdenv.isCygwin) ''
+    sed -i Makefile -e 's/-no-undefined//;'
+  '';
+
+  crossAttrs = {
+
+    # disable tests which can't run on build machine
+    postPatch = ''
+      substituteInPlace Makefile.in --replace "tests" " ";
+    '';
+
+    preConfigure = ''
+      export ac_cv_func_malloc_0_nonnull=yes
+      export ac_cv_func_realloc_0_nonnull=yes
+    '';
+  };
+
+  meta = {
+    homepage = https://github.com/westes/flex;
+    description = "A fast lexical analyser generator";
+    platforms = stdenv.lib.platforms.unix;
+  };
+}

pkgs/top-level/all-packages.nix

@@ -6276,6 +6276,7 @@ in
   jdepend = callPackage ../development/tools/analysis/jdepend { };
 
   flex_2_5_35 = callPackage ../development/tools/parsing/flex/2.5.35.nix { };
+  flex_2_6_1 = callPackage ../development/tools/parsing/flex/2.6.1.nix { };
   flex = callPackage ../development/tools/parsing/flex/default.nix { };
 
   flexcpp = callPackage ../development/tools/parsing/flexc++ { };