build: let SYCL to choose C++ standard

CMakeLists.txt

@@ -106,11 +106,6 @@ set(CMAKE_SRC_CCXX_FLAGS)       # SRC specifics
 set(CMAKE_EXAMPLE_CCXX_FLAGS)   # EXAMPLE specifics
 set(CMAKE_TEST_CCXX_FLAGS)      # TESTS specifics
 
-if(UNIX OR MINGW)
-    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -std=c99")
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
-endif()
-
 include("cmake/mkldnn_compat.cmake")
 
 include("cmake/utils.cmake")
@@ -130,6 +125,14 @@ include("cmake/coverage.cmake")
 include("cmake/build_types.cmake")
 include("cmake/testing.cmake")
 
+if(UNIX OR MINGW)
+    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -std=c99")
+    # Let SYCL to choose the C++ standard it needs.
+    if(NOT DNNL_WITH_SYCL)
+        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
+    endif()
+endif()
+
 # Handle cases when OpenMP runtime is requested but not found: override CPU
 # runtime to be sequential
 if(DNNL_CPU_RUNTIME STREQUAL "OMP" AND