scotch: make ptscotch optional

pkgs/by-name/mu/mumps/package.nix

···
 assert withParmetis -> mpiSupport;
 assert withPtScotch -> mpiSupport;
 let
+  scotch' = scotch.override { inherit withPtScotch; };
   profile = if mpiSupport then "debian.PAR" else "debian.SEQ";
   LMETIS = toString ([ "-lmetis" ] ++ lib.optional withParmetis "-lparmetis");
   LSCOTCH = toString (
···
       "LIBEXT_SHARED=.dylib"
     ]
     ++ [
-      "ISCOTCH=-I${lib.getDev scotch}/include"
+      "ISCOTCH=-I${lib.getDev scotch'}/include"
       "LMETIS=${LMETIS}"
       "LSCOTCH=${LSCOTCH}"
       "ORDERINGSF=${ORDERINGSF}"
···
       blas
       lapack
       metis
-      scotch
+      scotch'
     ];
 
   doInstallCheck = true;

pkgs/by-name/sc/scotch/package.nix

···
   xz,
   zlib,
   mpi,
+  withPtScotch ? false,
   testers,
 }:
 
···
 
   cmakeFlags = [
     (lib.cmakeBool "BUILD_SHARED_LIBS" (!stdenv.hostPlatform.isStatic))
+    (lib.cmakeBool "BUILD_PTSCOTCH" withPtScotch)
   ];
 
   nativeBuildInputs = [
···
     zlib
   ];
 
-  propagatedBuildInputs = [
+  propagatedBuildInputs = lib.optionals withPtScotch [
     mpi
   ];