$OpenBSD: patch-setupext_py,v 1.18 2019/05/01 11:38:52 bket Exp $
Index: setupext.py
--- setupext.py.orig
+++ setupext.py
@@ -1487,6 +1487,7 @@ class BackendTkAgg(OptionalBackendPackage):
             ]
 
         ext = make_extension('matplotlib.backends._tkagg', sources)
+        ext.include_dirs.extend(['${X11BASE}/include'])
         self.add_flags(ext)
         LibAgg().add_flags(ext, add_sources=False)
         return ext