syclstd 2017 in new release to propagate subgroup

CMakeLists.txt

@@ -67,7 +67,8 @@ if(MSVC)
 elseif(GINKGO_BUILD_DPCPP OR CMAKE_CXX_COMPILER MATCHES "dpcpp")
     # For now always use `-ffp-model=precise` with DPC++. This can be removed when
     # the floating point issues are fixed.
-    set(GINKGO_COMPILER_FLAGS "-Wpedantic;-ffp-model=precise" CACHE STRING
+    # -sycl-std=2017 is temporary workaround after 2021.4 to propagate subgroup setting correctly
+    set(GINKGO_COMPILER_FLAGS "-Wpedantic;-ffp-model=precise;-sycl-std=2017" CACHE STRING
         "Set the required CXX compiler flags, mainly used for warnings. Current default is `-Wpedantic;-ffp-model=precise`")
 else()
     set(GINKGO_COMPILER_FLAGS "-Wpedantic" CACHE STRING