yosys-bluespec: fix build with yosys 0.67

pkgs/development/compilers/yosys/plugins/bluespec.nix

@@ -21,6 +21,12 @@ stdenv.mkDerivation {
     sha256 = "sha256-3cNFP/k4JsgLyUQHWU10Htl2Rh0staAcA3R4piD6hDE=";
   };
 
+  # yosys >=0.67 headers require C++20; the Makefile otherwise pins c++17.
+  postPatch = ''
+    substituteInPlace Makefile \
+      --replace-fail '-std=c++17' '-std=c++20'
+  '';
+
   buildInputs = [
     yosys
     readline