Improve import code robustness

Author: sethrj

include/flc.i

@@ -7,6 +7,10 @@
 
 %module "flc"
 
+#if defined(SWIGIMPORTED) && !defined(SWIGIMPORTED)
+#error "To import the FLC module correctly, use ``%include \"import_flc.i\"``"
+#endif
+
 %define %flc_add_header
 %insert("fbegin") %{
 ! Flibcpp project, https://github.com/swig-fortran/flibcpp

include/import_flc.i

@@ -5,8 +5,10 @@
  * Distributed under an MIT open source license: see LICENSE for details.
  */
 
-#ifdef SWIGIMPORTED
-#error "To use this header, %include "import_flc.i", don't %import it"
+// Make sure that the downstream module isn't doing ``%import "import_flc.i"``
+// or ``%import "flc.i"``: only ``%include "import_flc.i"``
+#ifndef SWIGIMPORTED
+#define FLC_SWIGIMPORTED
 #endif
 
 // Set up macros, etc.