Enable GMX_GPU=SYCL compiles

Enable GMX_GPU=SYCL compiles

Update CMake config to properly apply -fsycl, but only to files
including actual SYCL code, and get linking working.  Tested
with both dpcpp and icpx, and added trivial SYCL code to a
dimmy device context, just to ensure compiles and linking works
both with and without SYCL support enabled.

Also fixes a number of new warnings about unused variables now
detected with dpcpp and icpx.