Find a source file in srcdir to allow to build outside of srcdir.

setup.py

@@ -863,7 +863,7 @@ class PyBuildExt(build_ext):
         # Fredrik Lundh's cElementTree module.  Note that this also
         # uses expat (via the CAPI hook in pyexpat).
 
-        if os.path.isfile('Modules/_elementtree.c'):
+        if os.path.isfile(os.path.join(srcdir, 'Modules', '_elementtree.c')):
             define_macros.append(('USE_PYEXPAT_CAPI', None))
             exts.append(Extension('_elementtree',
                                   define_macros = define_macros,