Fix cmake diagnostics of the used backend

[dmitriy-sobolev]

This fixes an issue when cmake provides wrong information on the used backend.

It happens when all the conditions are satisfied:

• The compiler supports -fsycl feature
• SYCL headers are not available
• ONEDPL_BACKEND is equal to dpcpp or dpcpp_only or not explicitly provided.

In that case, cmake prints a message that DPCPP backend is going to be used, but oneDPL headers do not enable it, because SYCL headers are not available.

Examples of such compilers: Clang++ v13, v14.

[danhoeflinger]

LGTM.
As a confirmation this feature (CHECK_INCLUDE_FILE_CXX) and the way we use it is available for CMake 3.11, so this does not impact our minimum CMake version requirements.

[dmitriy-sobolev]

I am going to merge it after the next release in order not to bring more risks.