pocl: add setupHook to specifies directory to scan for ICDs

Adds a setup hook that sets the OCL_ICD_VENDORS environment variable
to point to a directory containing ICD (Installable Client Driver) files.

These files define the available OpenCL backends, allowing other OpenCL
applications to detect and use the appropriate drivers in sandbox environment.

pkgs/by-name/po/pocl/package.nix

@@ -80,6 +80,8 @@ stdenv.mkDerivation (finalAttrs: {
 
   passthru.updateScript = nix-update-script { };
 
+  setupHook = ./setup-hook.sh;
+
   meta = {
     description = "A portable open source (MIT-licensed) implementation of the OpenCL standard";
     homepage = "http://portablecl.org";

pkgs/by-name/po/pocl/setup-hook.sh

@@ -0,0 +1,6 @@
+preCheckHooks+=('setupPoclCheck')
+preInstallCheckHooks+=('setupPoclCheck')
+
+setupPoclCheck () {
+  export OCL_ICD_VENDORS="@out@/etc/OpenCL/vendors"
+}