be able to skip compiling fermion instantiations altogether

examples/disable_examples_without_instantiations.h

@@ -0,0 +1,14 @@
+#pragma once
+
+#ifndef BUILD_FERMION_INSTANTIATIONS
+#include <iostream>
+
+int main(void) {
+  std::cout << "This build of Grid was configured to exclude fermion instantiations, "
+	    << "which this example relies on. "
+	    << "Please reconfigure and rebuild Grid with --enable-fermion-instantiations"
+	    << "to run this example."
+	    << std::endl;
+  return 1;
+}
+#endif