Change standard to c++14 to be compatible to tensorpipe's dependencies (#3712)

Co-authored-by: Jinjing Zhou <VoVAllen@users.noreply.github.com>

CMakeLists.txt

@@ -92,11 +92,10 @@ if(MSVC)
   endif()
 else(MSVC)
   include(CheckCXXCompilerFlag)
-  check_cxx_compiler_flag("-std=c++11"    SUPPORT_CXX11)
+  # tensorpipe's dependencies require C++14
+  check_cxx_compiler_flag("-std=c++14"    SUPPORT_CXX14)
   set(CMAKE_C_FLAGS "-O2 -Wall -fPIC ${CMAKE_C_FLAGS}")
-  # We still use c++11 flag in CPU build because gcc5.4 (our default compiler) is
-  # not fully compatible with c++14 feature.
-  set(CMAKE_CXX_FLAGS "-O2 -Wall -fPIC -std=c++11 ${CMAKE_CXX_FLAGS}")
+  set(CMAKE_CXX_FLAGS "-O2 -Wall -fPIC -std=c++14 ${CMAKE_CXX_FLAGS}")
   if(NOT APPLE)
     set(CMAKE_SHARED_LINKER_FLAGS "-Wl,--warn-common ${CMAKE_SHARED_LINKER_FLAGS}")
   endif(NOT APPLE)