$OpenBSD: patch-setupext_py,v 1.17 2018/10/29 00:28:50 daniel Exp $
Index: setupext.py
--- setupext.py.orig
+++ setupext.py
@@ -1547,6 +1547,7 @@ class BackendTkAgg(OptionalBackendPackage):
             ]
 
         ext = make_extension('matplotlib.backends._tkagg', sources)
+        ext.include_dirs.extend(['${X11BASE}/include'])
         self.add_flags(ext)
         Numpy().add_flags(ext)
         LibAgg().add_flags(ext, add_sources=False)
