$OpenBSD: patch-setupext_py,v 1.20 2021/02/22 22:46:09 daniel Exp $
Index: setupext.py
--- setupext.py.orig
+++ setupext.py
@@ -969,6 +969,7 @@ class BackendTkAgg(OptionalBackendPackage):
             ]
 
         ext = 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)
