Fix google benchmarks interaction with CXXFLAGS with -fsycl

[hjabird]

• The Google Benchmark code does not work if CMAKE_CXX_FLAGS include -fsycl due to conflicting C++ version requirements.
• This removes the external CXX flags for the Google Benchmark library build.
• Also update Google Benchmark version tag.

Checklist

Tick if relevant:

• [N/A] New files have a copyright
• [N/A] New headers have an include guards
• [N/A] API is documented with Doxygen
• New functionalities are tested
• [N/A] Tests pass locally
• [N/A] Files are clang-formatted

[Rbiessy]

Can you clarify which error do you get without this fix?

[hjabird]

You get the error:

/opt/intel/oneapi/compiler/2024.1/bin/icpx -DBENCHMARK_STATIC_DEFINE -DHAVE_POSIX_REGEX -DHAVE_STD_REGEX -DHAVE_STEADY_CLOCK -I/builds/hugh.bird/sycl-fft/build/_deps/googlebenchmark-src/include -I/builds/hugh.bird/sycl-fft/build/_deps/googlebenchmark-src/src -Ofast -ferror-limit=5 -fsycl -fsycl-targets=amdgcn-amd-amdhsa -Xsycl-target-backend=amdgcn-amd-amdhsa --offload-arch=gfx1030  -Wall  -Wextra  -Wshadow  -Wfloat-equal  -Wsuggest-override  -pedantic  -pedantic-errors  -Wshorten-64-to-32  -fstrict-aliasing  -Wno-deprecated-declarations  -Wstrict-aliasing  -Wthread-safety -O3 -DNDEBUG -std=c++11 -fvisibility=hidden -fvisibility-inlines-hidden -pthread -MD -MT _deps/googlebenchmark-build/src/CMakeFiles/benchmark.dir/benchmark.cc.o -MF _deps/googlebenchmark-build/src/CMakeFiles/benchmark.dir/benchmark.cc.o.d -o _deps/googlebenchmark-build/src/CMakeFiles/benchmark.dir/benchmark.cc.o -c /builds/hugh.bird/sycl-fft/build/_deps/googlebenchmark-src/src/benchmark.cc
In file included from /builds/hugh.bird/sycl-fft/build/_deps/googlebenchmark-src/src/benchmark.cc:17:
In file included from /builds/hugh.bird/sycl-fft/build/_deps/googlebenchmark-src/src/benchmark_api_internal.h:4:
In file included from /opt/intel/oneapi/compiler/2024.1/bin/compiler/../../include/sycl/stl_wrappers/cmath:32:
/opt/intel/oneapi/compiler/2024.1/bin/compiler/../../include/sycl/detail/defines_elementary.hpp:81:15: error: static assertion failed due to requirement '201103L >= 201703L': DPCPP does not support C++ version earlier than C++17.
   81 | static_assert(__cplusplus >= 201703L,
      |               ^~~~~~~~~~~~~~~~~~~~~~
<built-in>:829:21: note: expanded from macro '__cplusplus'
  829 | #define __cplusplus 201103L

when adding extra cmake flags, eg:

export CXXFLAGS="-Ofast -ferror-limit=5 -fsycl -fsycl-targets=amdgcn-amd-amdhsa -Xsycl-target-backend=amdgcn-amd-amdhsa --offload-arch=gfx1030"

[Rbiessy]

I don't understand why are we facing this issue now? This used to build fine. Is this a global issue with DPC++ and Google Benchmark?
I can see their CMake checks the compiler ID for some flags so maybe we could create a PR in Google Benchmark if needed?

[hjabird]

After some discussion this probably isn't the best path forward. In the short-term, we have a work-around for where this is a problem for us. If this becomes a problem in another context, other alternatives can be explored, eg. PORTFFT_SYCL_FLAGS or something.