examples/fake-signing: Make runnable on Linux

The main reason for fake signing is making it run on Linux - and not
depend on `dxil.dll` on Windows - so it's great if this example runs on
Linux as well to see it all working even if that requires a few nasty
cfg blocks.  Note that this is done with `cfg!()` and regular `if`
blocks instead of attributes, such that this code lints and compiles on
both Windows and Linux even if the `validate_dxil()` codepath is never
entered on the latter (and that way includes, `let mut all_matches` etc
don't need to be `#[cfg(windows)]`'ed out either).

examples/validate-fake-signing.rs

@@ -26,27 +26,38 @@ fn main() {
 
         let without_digest = get_digest(&dxil);
 
-        let validated_dxil = validate_dxil(&dxil).unwrap();
-
-        let with_digest = get_digest(&validated_dxil);
-
         let result = fake_sign_dxil_in_place(&mut dxil);
         assert!(result);
 
         let fake_signed_digest = get_digest(&dxil);
 
-        println!(
-            "\tAfter compilation: {:?}\n\tAfter dxil.dll: {:?}\n\tAfter fake signing: {:?}",
-            without_digest, with_digest, fake_signed_digest
-        );
-
-        if fake_signed_digest != with_digest {
-            println!("---- Mismatch in file {} ----", idx);
-            all_matches &= false;
+        if cfg!(windows) {
+            let validated_dxil = validate_dxil(&dxil).unwrap();
+
+            let with_digest = get_digest(&validated_dxil);
+
+            println!(
+                "\tAfter compilation: {:?}\n\tAfter dxil.dll: {:?}\n\tAfter fake signing: {:?}",
+                without_digest, with_digest, fake_signed_digest
+            );
+
+            if fake_signed_digest != with_digest {
+                println!("---- Mismatch in file {} ----", idx);
+                all_matches &= false;
+            }
+        } else {
+            println!(
+                "\tAfter compilation: {:?}\n\tAfter fake signing: {:?}",
+                without_digest, fake_signed_digest
+            );
         }
     }
 
-    if all_matches {
-        println!("Success");
+    if cfg!(windows) {
+        if all_matches {
+            println!("Success");
+        }
+    } else {
+        println!("Warning: Signatures not validated against `dxil.dll` - this is only possible on Windows");
     }
 }