be able to skip compiling fermion instantiations altogether

tests/Test_dwf_mixedcg_prec_halfcomms.cc

@@ -25,6 +25,9 @@ Author: Peter Boyle <paboyle@ph.ed.ac.uk>
     See the full license in the file "LICENSE" in the top level distribution directory
     *************************************************************************************/
     /*  END LEGAL */
+#include "disable_tests_without_instantiations.h"
+#ifdef ENABLE_FERMION_INSTANTIATIONS
+
 #include <Grid/Grid.h>
 
 using namespace std;
@@ -118,3 +121,4 @@ int main (int argc, char ** argv)
   Grid_finalize();
 }
 #endif
+#endif