Revert "[libcxx] Enable C++17 for the benchmarks."

It seems several build bots have issues with setting the CXX_STANDARD
property to 17.

This reverts commit d184d02263.

libcxx/benchmarks/CMakeLists.txt

@@ -72,8 +72,18 @@ set(BENCHMARK_OUTPUT_DIR ${CMAKE_CURRENT_BINARY_DIR})
 set(BENCHMARK_LIBCXX_INSTALL ${CMAKE_CURRENT_BINARY_DIR}/benchmark-libcxx)
 set(BENCHMARK_NATIVE_INSTALL ${CMAKE_CURRENT_BINARY_DIR}/benchmark-native)
 
+check_flag_supported("-std=c++17")
+mangle_name("LIBCXX_SUPPORTS_STD_EQ_c++17_FLAG" BENCHMARK_SUPPORTS_STD_CXX17_FLAG)
+if (${BENCHMARK_SUPPORTS_STD_CXX17_FLAG})
+  set(BENCHMARK_DIALECT_FLAG "-std=c++17")
+else()
+  # If the compiler doesn't support -std=c++17, attempt to fall back to -std=c++1z while still
+  # requiring C++17 language features.
+  set(BENCHMARK_DIALECT_FLAG "-std=c++1z")
+endif()
+
 set(BENCHMARK_TEST_COMPILE_FLAGS
-    -O2
+    ${BENCHMARK_DIALECT_FLAG} -O2
     -fsized-deallocation
     -I${BENCHMARK_LIBCXX_INSTALL}/include
     -I${LIBCXX_SOURCE_DIR}/test/support
@@ -141,7 +151,6 @@ function(add_benchmark_test name source_file)
           OUTPUT_NAME "${name}.libcxx.out"
           RUNTIME_OUTPUT_DIRECTORY "${BENCHMARK_OUTPUT_DIR}"
           COMPILE_FLAGS "${BENCHMARK_TEST_LIBCXX_COMPILE_FLAGS}"
-          CXX_STANDARD 17
           LINK_FLAGS "${BENCHMARK_TEST_LIBCXX_LINK_FLAGS}")
   cxx_link_system_libraries(${libcxx_target})
   if (LIBCXX_BENCHMARK_NATIVE_STDLIB)