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

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[mkrainiuk]

Thank you for the fix! I have one minor comment but overall looks good to me.

[mkrainiuk]

This approach could not work correctly for case when MKLConfig is used for building non-SYCL app with clang compiler that supports SYCL, but I believe this fix is enough for oneMKL project build. We can update this part later with more general fix from Intel oneMKL

[dnhsieh-intel]

Thank you for the PR! I have some suggestions.

[dnhsieh-intel]

Suggested change

	include(CMakePackageConfigHelpers)
	include(CheckCXXCompilerFlag)
	include(CheckIncludeFileCXX)
	include(GNUInstallDirs)
	include(CheckCXXCompilerFlag)
	include(CheckIncludeFileCXX)

I think we can delete the first line include(CMakePackageConfigHelpers) and the fourth line include(GNUInstallDirs). CheckCXXCompilerFlag is needed for check_cxx_compiler_flag(), and CheckIncludeFileCXX is needed for CHECK_INCLUDE_FILE_CXX().

[dnhsieh-intel]

Could you put this block in a condition please?

if("CXX" IN_LIST CURR_LANGS)
  include(CheckCXXCompilerFlag)
  include(CheckIncludeFileCXX)

  check_cxx_compiler_flag("-fsycl" _fsycl_option)
  ...
      if (_sycl_header OR _sycl_header_old)
      set(SYCL_COMPILER ON)
    endif()
  endif()
endif()

Although only CXX is used in oneMKL interfaces, oneMKL also needs to support C and Fortran. There is a possibility that users would use MKLConfig.cmake here with oneMKL.

[dnhsieh-intel]

Did you mean

Suggested change

	if(NOT DEFINED SYCL_COMPILER OR SYCL_COMPILER MATCHES OFF)
	if(NOT SYCL_COMPILER)

[dnhsieh-intel]

SYCL_SYCL_HPP and CL_SYCL_HPP might be more expressive than _sycl_header and _sycl_header_old. It would be great if you also use COMPILER_FSYCL_SUPPORT instead of _fsycl_option. Please note that the convention in MKLConfig.cmake is to use all caps for variable names.

In addition, please consider removing the space after if to match the style in the file:

if(<condition>)