Add C++ Z4C RHS path and port some BSSN optimizations

AMSS_NCKU_source/makefile.inc

@@ -48,6 +48,11 @@ endif
 ##   0           : fall back to original Fortran kernels
 USE_CXX_KERNELS ?= 1
 
+## Z4C Cartesian RHS kernel switch
+##   1 (default) : use C++ rewrite of Z4c_rhs (main Cartesian path faster)
+##   0           : use original Fortran Z4c_rhs.o
+USE_CXX_Z4C_KERNELS ?= 1
+
 ## RK4 kernel implementation switch
 ##   1 (default) : use C/C++ rewrite of rungekutta4_rout (for optimization experiments)
 ##   0           : use original Fortran rungekutta4_rout.o