diff --git a/src/bindings/python/bt2/Makefile.am b/src/bindings/python/bt2/Makefile.am
index 20f142e4e..b74821ae8 100644
--- a/src/bindings/python/bt2/Makefile.am
+++ b/src/bindings/python/bt2/Makefile.am
@@ -120,6 +120,14 @@ WARN_CFLAGS += -Wno-deprecated-declarations
 # Python 3.11 + gcc 12.2.0 gave warnings of this kind in Python.h.
 WARN_CFLAGS += -Wno-redundant-decls
 
+# SWIG 4.2.0 generates:
+#
+#   #if __cplusplus >=201103L
+#
+# ... leading to some `-Wundef` warnings when building the extension as C, where
+# `__cplusplus` is not defined.
+WARN_CFLAGS += -Wno-undef
+
 BUILD_FLAGS=CC="$(CC)" \
 		CFLAGS="$(GLIB_CFLAGS) $(AM_CFLAGS) $(CFLAGS) $(WARN_CFLAGS)" \
 		CPPFLAGS="$(AM_CPPFLAGS) $(CPPFLAGS) -I$(srcdir)/bt2" \