Fix TriSycl CMake files.

This is to enable compiling with the latest trisycl. FindTriSYCL.cmake was broken by commit 00f32752, which modified add_sycl_to_target for ComputeCPP. This makes the corresponding modifications for trisycl to make them consistent.

trisycl also now requires c++17.

Note: the code doesn't actually compile with trisycl due to errors in the sycl backend, but this does fix the build configuration itself.