Update SYCL support discovery to check for `-fsycl` flag.

[BenBrock]

Description

oneMKL's CMake currently checks CXX_COMPILER_NAME and uses that to determine whether SYCL is supported. This prevents SYCL support from being detected if the user compiles with their own build of intel/llvm (see issue #449).

This is a first stab at modifying MKLConfig.cmake to check for -fsycl support directly instead of inferring from the compiler name. It is based on how oneDPL checks for SYCL support, which works for both release builds of IntelLLVM and custom builds of intel/llvm.

I have also modified the CMake to avoid setting CLANG_COMPILER if SYCL_COMPILER is set, as this is the behavior when using IntelLLVM. I assume open-source DPC++ builds should behave similarly.

Fixes #449

Checklist

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