ffado: support cross compilation (#334096)

pkgs/os-specific/linux/ffado/default.nix

@@ -49,13 +49,25 @@ stdenv.mkDerivation rec {
   prePatch = ''
     substituteInPlace ./support/tools/ffado-diag.in \
       --replace /lib/modules/ "/run/booted-system/kernel-modules/lib/modules/"
+
+    # prevent build tools from leaking into closure
+    substituteInPlace support/tools/SConscript --replace-fail \
+      'support/tools/ffado-diag --static' \
+      "echo '"'See `nix-store --query --tree ${placeholder "out"}`.'"'"
+  '' + lib.optionalString (!stdenv.buildPlatform.canExecute stdenv.hostPlatform) ''
+    # skip the CC sanity check, since that requires invoking cross-compiled binaries during build
+    substituteInPlace SConstruct \
+      --replace-fail 'conf.CompilerCheck()' 'True' \
+      --replace-fail "pkg-config" "$PKG_CONFIG"
+    substituteInPlace admin/pkgconfig.py \
+      --replace-fail "pkg-config" "$PKG_CONFIG"
   '';
 
   nativeBuildInputs =
     [
       (scons.override {
         # SConstruct script depends on distutils removed in Python 3.12
-        python3Packages = python311.pkgs;
+        python3Packages = python311.pythonOnBuildForHost.pkgs;
       })
       pkg-config
       which
@@ -68,6 +80,8 @@ stdenv.mkDerivation rec {
 
   prefixKey = "PREFIX=";
   sconsFlags = [
+    "CUSTOM_ENV=True"  # tell SConstruct to use nixpkgs' CC/CXX/CFLAGS
+    "DETECT_USERSPACE_ENV=False"
     "DEBUG=False"
     "ENABLE_ALL=True"
     "BUILD_TESTS=True"
@@ -97,11 +111,7 @@ stdenv.mkDerivation rec {
 
   enableParallelBuilding = true;
   dontWrapQtApps = true;
-
-  postInstall = ''
-    # prevent build tools from leaking into closure
-    echo 'See `nix-store --query --tree ${placeholder "out"}`.' > $out/lib/libffado/static_info.txt
-  '';
+  strictDeps = true;
 
   preFixup = lib.optionalString withMixer ''
     wrapQtApp "$bin/bin/ffado-mixer"