The cpp20 dir doesn't exist
https://github.com/chriskohlhoff/asio/issues/1026

--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -15,7 +15,6 @@ EXAMPLES_CPP17 = examples/cpp17
 endif
 
 if HAVE_CXX20
-EXAMPLES_CPP20 = examples/cpp20
 endif
 
 SUBDIRS = \
@@ -23,7 +22,6 @@ SUBDIRS = \
 	$(EXAMPLES_CPP11) \
 	$(EXAMPLES_CPP14) \
 	$(EXAMPLES_CPP17) \
-	$(EXAMPLES_CPP20) \
 	tests
 
 DIST_SUBDIRS = examples/cpp03 examples/cpp11 examples/cpp14 examples/cpp17 tests
--- a/configure.ac
+++ b/configure.ac
@@ -241,4 +241,4 @@ AC_OUTPUT([
   src/examples/cpp11/Makefile
   src/examples/cpp14/Makefile
   src/examples/cpp17/Makefile
-  src/examples/cpp20/Makefile])
+  ])