backends: avoid extraneous trailing os.path.sep when joining paths

resolves #2336

mesonbuild/backend/backends.py

@@ -162,8 +162,11 @@ class Backend:
         return os.path.relpath(target_dir, othert_dir)
 
     def get_target_source_dir(self, target):
-        dirname = os.path.join(self.build_to_src, self.get_target_dir(target))
-        return dirname
+        # if target dir is empty, avoid extraneous trailing / from os.path.join()
+        target_dir = self.get_target_dir(target)
+        if target_dir:
+            return os.path.join(self.build_to_src, target_dir)
+        return self.build_to_src
 
     def get_target_private_dir(self, target):
         dirname = os.path.join(self.get_target_dir(target), target.get_basename() + target.type_suffix())