scid-vs-pc: fix build with Tcl stubs default

scid-vs-pc uses a custom configure script rather than a TEA configure
script, so it rejects the --enable-stubs flag now added by
tcl.mkTclDerivation. Disable the default Tcl configure flags for this
package and pass only the Tcl library/include paths and exec prefix that
upstream's configure accepts.

Keep the dependency argument as libx11, matching the package name exposed
by nixpkgs.

Tested: nix build --no-link --print-out-paths github:johnrichardrinehart/nixpkgs/fix-scid-vs-pc-tcl-stubs#scid-vs-pc

pkgs/by-name/sc/scid-vs-pc/package.nix

@@ -35,9 +35,13 @@ tcl.mkTclDerivation rec {
     zlib
   ];
 
+  addTclConfigureFlags = false;
   configureFlags = [
     "BINDIR=${placeholder "out"}/bin"
     "SHAREDIR=${placeholder "out"}/share"
+    "--with-tcl=${tcl}/lib"
+    "--with-tclinclude=${tcl}/include"
+    "--exec-prefix=${placeholder "out"}"
   ];
 
   postInstall = ''