[OpenMP] always compile with c++14 instead of gnu++14

Fixes PR 51174. c++14 should be a more portable option than gnu++14.

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D106632

Author: ye-luo

openmp/cmake/HandleOpenMPOptions.cmake

@@ -29,7 +29,4 @@ append_if(OPENMP_HAVE_WNO_EXTRA_FLAG "-Wno-extra" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
 append_if(OPENMP_HAVE_WNO_PEDANTIC_FLAG "-Wno-pedantic" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
 append_if(OPENMP_HAVE_WNO_MAYBE_UNINITIALIZED_FLAG "-Wno-maybe-uninitialized" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
 
-append_if(OPENMP_HAVE_STD_GNUPP14_FLAG "-std=gnu++14" CMAKE_CXX_FLAGS)
-if (NOT OPENMP_HAVE_STD_GNUPP14_FLAG)
-  append_if(OPENMP_HAVE_STD_CPP14_FLAG "-std=c++14" CMAKE_CXX_FLAGS)
-endif()
+append_if(OPENMP_HAVE_STD_CPP14_FLAG "-std=c++14" CMAKE_CXX_FLAGS)

openmp/cmake/config-ix.cmake

@@ -14,5 +14,4 @@ check_cxx_compiler_flag(-Wno-extra OPENMP_HAVE_WNO_EXTRA_FLAG)
 check_cxx_compiler_flag(-Wno-pedantic OPENMP_HAVE_WNO_PEDANTIC_FLAG)
 check_cxx_compiler_flag(-Wno-maybe-uninitialized OPENMP_HAVE_WNO_MAYBE_UNINITIALIZED_FLAG)
 
-check_cxx_compiler_flag(-std=gnu++14 OPENMP_HAVE_STD_GNUPP14_FLAG)
 check_cxx_compiler_flag(-std=c++14 OPENMP_HAVE_STD_CPP14_FLAG)