be able to skip compiling fermion instantiations altogether

examples/Example_wall_wall_3pt.cc

@@ -3,6 +3,9 @@
  * without regression / tests being applied
  */
 
+#include "disable_examples_without_instantiations.h"
+#ifdef ENABLE_FERMION_INSTANTIATIONS
+
 #include <Grid/Grid.h>
 
 using namespace std;
@@ -535,5 +538,4 @@ int main (int argc, char ** argv)
   Grid_finalize();
 }
 
-
-
+#endif