Add unit test for ifndef

See https://git.dynare.org/Dynare/dynare/-/issues/1747

tests/Makefile.am

@@ -81,6 +81,7 @@ MODFILES = \
 	example2long.mod \
 	example2long_use_dll.mod \
 	t_sgu_ex1.mod \
+	macro_processor/test_ifndef.mod \
 	irfs/example1_unit_std.mod \
 	optimal_policy/OSR/osr_example.mod \
 	optimal_policy/OSR/osr_example_objective_correctness.mod \

tests/macro_processor/test_ifndef.mod

@@ -0,0 +1,17 @@
+// test ifndef to prevent regressions like #1747
+@#define marco=1
+@#ifndef marco
+    def=0;
+@#else
+    def=1;
+@#endif
+
+if ~def
+    error('ifndef not reached')
+end
+
+var x;
+
+model;
+x;
+end;