Fix Builder on cfg(not(any(unix, windows)))

- cfg(target_arch = "wasm32") would panic if you tried to do any std::fs
  Builder stuff, which was fine for wasm32-unknown-unknown and
  wasm32-unknown-emscripten, but not good for wasm32-wasi, which has a
  file system.

  (Might still want a specialised implementation for wasm32-wasi:
  std::os::wasi::fs::FileTypeExt is currently unstable, but offers
  is_block_device, is_character_device, is_socket_dgram and
  is_socket_stream. But this one is good to start with.)

- cfg(not(any(unix, windows, target_arch = "wasm32"))) would fail to
  compile (… unless you disabled the builder feature, since my previous
  commit).

Now it all works pretty decently.

src/header.rs

@@ -736,12 +736,6 @@ impl Header {
         }
     }
 
-    #[cfg(all(target_arch = "wasm32", feature = "builder"))]
-    #[allow(unused_variables)]
-    fn fill_platform_from(&mut self, meta: &fs::Metadata, mode: HeaderMode) {
-        unimplemented!();
-    }
-
     #[cfg(all(unix, feature = "builder"))]
     fn fill_platform_from(&mut self, meta: &fs::Metadata, mode: HeaderMode) {
         match mode {
@@ -801,39 +795,53 @@ impl Header {
         }
     }
 
-    #[cfg(all(windows, feature = "builder"))]
+    #[cfg(all(not(unix), feature = "builder"))]
+    #[allow(unused_variables)]
     fn fill_platform_from(&mut self, meta: &fs::Metadata, mode: HeaderMode) {
-        // There's no concept of a file mode on Windows, so do a best approximation here.
-        match mode {
+        // Fallback implementation: approximate modes, and get as good an mtime as we can.
+        let mtime = match mode {
+            #[cfg(windows)]
             HeaderMode::Complete => {
-                self.set_uid(0);
-                self.set_gid(0);
                 // The dates listed in tarballs are always seconds relative to
                 // January 1, 1970. On Windows, however, the timestamps are returned as
                 // dates relative to January 1, 1601 (in 100ns intervals), so we need to
                 // add in some offset for those dates.
-                let mtime = (meta.last_write_time() / (1_000_000_000 / 100)) - 11644473600;
-                self.set_mtime(mtime);
-                let fs_mode = {
-                    const FILE_ATTRIBUTE_READONLY: u32 = 0x00000001;
-                    let readonly = meta.file_attributes() & FILE_ATTRIBUTE_READONLY;
-                    match (meta.is_dir(), readonly != 0) {
-                        (true, false) => 0o755,
-                        (true, true) => 0o555,
-                        (false, false) => 0o644,
-                        (false, true) => 0o444,
-                    }
-                };
-                self.set_mode(fs_mode);
+                (meta.last_write_time() / (1_000_000_000 / 100)) - 11644473600
             }
-            HeaderMode::Deterministic => {
-                self.set_uid(0);
-                self.set_gid(0);
-                self.set_mtime(123456789); // see above in unix
-                let fs_mode = if meta.is_dir() { 0o755 } else { 0o644 };
-                self.set_mode(fs_mode);
+            #[cfg(not(windows))]
+            HeaderMode::Complete => {
+                meta.modified()
+                    .ok()
+                    .and_then(|time| time.duration_since(std::time::SystemTime::UNIX_EPOCH).ok())
+                    .map(|duration| duration.as_secs())
+                    .unwrap_or(1) // avoiding zero mtimes, see above in unix
             }
-        }
+            HeaderMode::Deterministic => 123456789, // see above in unix
+        };
+        let fs_mode = match mode {
+            #[cfg(windows)]
+            HeaderMode::Complete => {
+                const FILE_ATTRIBUTE_READONLY: u32 = 0x00000001;
+                let readonly = meta.file_attributes() & FILE_ATTRIBUTE_READONLY;
+                match (meta.is_dir(), readonly != 0) {
+                    (true, false) => 0o755,
+                    (true, true) => 0o555,
+                    (false, false) => 0o644,
+                    (false, true) => 0o444,
+                }
+            }
+            _ => {
+                if meta.is_dir() {
+                    0o755
+                } else {
+                    0o644
+                }
+            }
+        };
+        self.set_uid(0);
+        self.set_gid(0);
+        self.set_mtime(mtime);
+        self.set_mode(fs_mode);
 
         let ft = meta.file_type();
         self.set_entry_type(if ft.is_dir() {