groff: add missing bison dependency for Clang

The condition here is the same used to apply a patch above that makes
this dependency necessary.

I'm not sure why this isn't needed on Darwin, but it's definitely
needed for building on Linux with pkgsLLVM or clangStdenv, and for
FreeBSD.

Normally I'd make the patch application unconditional to prevent
situations like this ending up unnoticed, but since it requires an
extra dependency and will be fixed in the next release anyway, let's
just leave it.

pkgs/tools/text/groff/default.nix

@@ -5,6 +5,7 @@
 , autoreconfHook
 , pkg-config
 , texinfo
+, bison
 , bash
 }:
 
@@ -51,7 +52,9 @@ stdenv.mkDerivation rec {
   '';
 
   strictDeps = true;
-  nativeBuildInputs = [ autoreconfHook pkg-config texinfo ];
+  nativeBuildInputs = [ autoreconfHook pkg-config texinfo ]
+    # Required due to the patch that changes .ypp files.
+    ++ lib.optional (stdenv.cc.isClang && lib.versionAtLeast stdenv.cc.version "9") bison;
   buildInputs = [ perl bash ]
     ++ lib.optionals enableGhostscript [ ghostscript ]
     ++ lib.optionals enableHtml [ psutils netpbm ];