voms: default to symlink $out/etc to /etc

pkgs/tools/networking/voms/default.nix

@@ -11,6 +11,10 @@
 , gsoap
 , openssl
 , zlib
+  # Configuration overridable with .override
+  # If not null, the builder will
+  # move "$out/etc" to "$out/etc.orig" and symlink "$out/etc" to externalEtc.
+, externalEtc ? "/etc"
 }:
 
 stdenv.mkDerivation rec{
@@ -24,6 +28,10 @@ stdenv.mkDerivation rec{
     hash = "sha256-FG4fHO2lsQ3t/ZaKT9xY+xqdQHfdtzi5ULtxLhdPnss=";
   };
 
+  passthru = {
+    inherit externalEtc;
+  };
+
   nativeBuildInputs = [
     autoreconfHook
     bison
@@ -59,6 +67,13 @@ stdenv.mkDerivation rec{
     "--with-gsoap-wsdl2h=${gsoap}/bin/wsdl2h"
   ];
 
+  postFixup = ''
+    ${lib.optionalString (externalEtc != null) ''
+      mv "$out"/etc{,.orig}
+      ln -s ${lib.escapeShellArg externalEtc} "$out/etc"
+    ''}
+  '';
+
   meta = with lib; {
     description = "The C/C++ VOMS server, client and APIs v2.x";
     homepage = "https://italiangrid.github.io/voms/";