_4th: fix editor for 64bit systems

Use headers in `sources/include{32, 64}` to fix the builtin editor, as suggested by `README`.
The problem is described in the FAQ section of the [manual](https://thebeez.home.xs4all.nl/4tH/4tHmanual.pdf).
Although the manual suggests regenerating the headers (section 27.8), the required headers seem to be already in the distirbution.

pkgs/by-name/_4/_4th/package.nix

@@ -20,6 +20,11 @@ stdenv.mkDerivation (finalAttrs: {
 
   dontConfigure = true;
 
+  preBuild = ''
+    cp sources/include${if stdenv.hostPlatform.is64bit then "64" else "32"}/* sources/
+    make -C sources clean
+  '';
+
   makeFlags = [
     "-C sources"
     "CC:=$(CC)"