arcan: create a wrapper

This commit creates a wrapper with the purpose of gracefully execute Arcan
applications (appls, in Arcan jargon).

In a typical FHS system, Arcan can be invoked by a commandline like `arcan
<appl>`, or more generally `arcan <arcan options> <appl> <appl options>`. In
order to emulate this behaviour, this wrapper was crafted.

It `symlinkJoin`s Arcan and the appls, and sets the relevant environment
variables accordingly.

pkgs/desktops/arcan/default.nix

@@ -10,4 +10,5 @@ rec {
   # Arcan
 
   arcan = callPackage ./arcan.nix { };
+  arcan-wrapped = callPackage ./wrapper.nix { };
 }

pkgs/desktops/arcan/wrapper.nix

@@ -0,0 +1,28 @@
+{ arcan
+, makeWrapper
+, symlinkJoin
+, appls ? [ ]
+, name ? "arcan-wrapped"
+}:
+
+symlinkJoin rec {
+  inherit name;
+
+  paths = appls ++ [ arcan ];
+
+  nativeBuildInputs = [ makeWrapper ];
+
+  postBuild = ''
+    for prog in ${placeholder "out"}/bin/*; do
+      wrapProgram $prog \
+        --prefix PATH ":" "${placeholder "out"}/bin" \
+        --set ARCAN_APPLBASEPATH "${placeholder "out"}/share/arcan/appl/" \
+        --set ARCAN_BINPATH "${placeholder "out"}/bin/arcan_frameserver" \
+        --set ARCAN_LIBPATH "${placeholder "out"}/lib/" \
+        --set ARCAN_RESOURCEPATH "${placeholder "out"}/share/arcan/resources/" \
+        --set ARCAN_SCRIPTPATH "${placeholder "out"}/share/arcan/scripts/" \
+        --set ARCAN_STATEBASEPATH "$HOME/.arcan/resources/savestates/"
+    done
+  '';
+}
+# TODO: set ARCAN_FONTPATH to a set of fonts that can be provided in a parameter