Fractal SasView models (and potentially further ones) need -DUSE_CUDA in case of OPENACC

[willend]

It works to add a small patch at the top of sas_kernel_header.c like this and attached:

#ifdef OPENACC
# define USE_CUDA
#endif

sas_kernel_header.c.txt

But perhaps we should do this at the McCode level instead? ... If there are no immediate side-effects? Alongside #include <accelmath.h> perhaps?

@farhi any point of view? @jorobledo discovered the symptoms and I found the underlying issue in sas_kernel_header.c