bt2: compile native_bt.c with -Wno-undef

As explained in the comment, work around a problem with SWIG 4.2.0.

Change-Id: Ia95fc1e4929591a0a16df48892b05f5e5df46be3
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11849
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Michael Jeanson <mjeanson@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11930
Reviewed-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>

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" \