LIBCXX related fixes

[yamt]

Summary

LIBCXX related fixes

Impact

Testing

See #2063

[jerpelea]

LGTM

[xiaoxiang781216]

@yamt do we still need this PR? I am asking this question is that most change has been moved to other PR now.

[yamt]

@xiaoxiang781216 do you still have any concern?

[xiaoxiang781216]

The only concern is that "CXXFLAGS += -std=c++14 -D_LIBCPP_STD_VER=14", should we keep the default one from gcc/clang?

[yamt]

The only concern is that "CXXFLAGS += -std=c++14 -D_LIBCPP_STD_VER=14", should we keep the default one from gcc/clang?

ok, let me try to remove it

[xiaoxiang781216]

LGTM.

[yamt]

The only concern is that "CXXFLAGS += -std=c++14 -D_LIBCPP_STD_VER=14", should we keep the default one from gcc/clang?

ok, let me try to remove it

i remembered why it was necessary.
libcxx needs newer c++ version than the default of my compiler.
do you have a suggestion better than this?

spacetanuki% g++ -dM -E -x c++ /dev/null | grep __cplusplus
#define __cplusplus 199711L
spacetanuki% g++ --version                                 
Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/4.2.1
Apple clang version 11.0.0 (clang-1100.0.33.17)
Target: x86_64-apple-darwin18.7.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin
spacetanuki% 

[xiaoxiang781216]

From this link:
https://stackoverflow.com/questions/21581838/set-as-default-c11-in-clang
clang should target c++11 by default from 6.0.0.

[yamt]

From this link:
https://stackoverflow.com/questions/21581838/set-as-default-c11-in-clang
clang should target c++11 by default from 6.0.0.

what's your point?

[xiaoxiang781216]

My point is that it's strange why your version still default to c++98.

[yamt]

My point is that it's strange why your version still default to c++98.

ok. i agree it's strange. maybe apple is more picky about compatibility than the upstream?