quda backends

[simone-romiti]

(quoting @kostrzewa from the AMD workshop's QUDA chat)

We should probably do two things:

1. finally remove the --enable-gpu flag and all the gpu stuff that Florian added about a decade ago. (we can do this later though, not now)
2. add the option to link against QUDA with different backends

if you look at configure.in line 964, this is where the -lcuda -lcublas linking flags come from. This section should be taken outside the --with-qudadir option and made stand-alone.

Then, one will be able to do something like --with-qudadir=[...] --with-cudadir=[...] for a CUDA build,
while doing something like --with-qudadir=[...] --with-hipdir=[...] for the HIP build.

[simone-romiti]

I created a branch to apply this change. Please see #541