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.
Also, trisycl now requires c++17.
Ancient versions of CMake required else(), endif(), and similar block
termination commands to have arguments matching the command starting the block.
This is no longer the preferred style.