Do not expect a compiler to define SYCL_LANGUAGE_VERSION

[dmitriy-sobolev]

SYCL specification allows SYCL to be implemented purely as a library: https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html#_library_only_implementation. It means that inclusion of sycl/sycl.hpp is a portable way to get SYCL_LANGUAGE_VERSION, in contrast to expecting it to be defined by the compiler itself. Additionally, AdaptiveCPP defines this macro in sycl/sycl.hpp.