python3Packages.afdko: move env vars into env for structuredAttrs (#492263)

pkgs/development/python-modules/afdko/default.nix

@@ -70,10 +70,16 @@ buildPythonPackage (finalAttrs: {
     })
   ];
 
-  env.NIX_CFLAGS_COMPILE = lib.optionalString stdenv.cc.isClang (toString [
-    "-Wno-error=incompatible-function-pointer-types"
-    "-Wno-error=int-conversion"
-  ]);
+  env = {
+    # Use system libxml2
+    FORCE_SYSTEM_LIBXML2 = true;
+  }
+  // lib.optionalAttrs stdenv.cc.isClang {
+    NIX_CFLAGS_COMPILE = toString [
+      "-Wno-error=incompatible-function-pointer-types"
+      "-Wno-error=int-conversion"
+    ];
+  };
 
   # setup.py will always (re-)execute cmake in buildPhase
   dontConfigure = true;
@@ -95,9 +101,6 @@ buildPythonPackage (finalAttrs: {
   ++ fonttools.optional-dependencies.unicode
   ++ fonttools.optional-dependencies.woff;
 
-  # Use system libxml2
-  FORCE_SYSTEM_LIBXML2 = true;
-
   nativeCheckInputs = [ pytestCheckHook ];
 
   preCheck = ''