Fix deleted line.

Cython/Build/Dependencies.py

@@ -966,6 +966,7 @@ def cythonize(module_list, exclude=None, nthreads=0, aliases=None, quiet=False,
                 if build_dir:
                     if os.path.isabs(c_file):
                       warnings.warn("build_dir has no effect for absolute source paths")
+                    c_file = os.path.join(build_dir, c_file)
                     dir = os.path.dirname(c_file)
                     safe_makedirs_once(dir)