Feature/detect libcxx

[memsharded]

Changelog: Feature: Implement real detection of compiler.libcxx value for gcc compiler. Only enabled in CONAN_V2_MODE, otherwise it would be breaking.
Docs: Omit

Close #7264

[SSE4]

static_assert is C++11, can we use it?

[SSE4]

maybe we can just include and check the value of _GLIBCXX_USE_CXX11_ABI?

[memsharded]

It is only applied for gcc>=5.1 which already support c++11 (passed in the command line)

maybe we can just include and check the value of _GLIBCXX_USE_CXX11_ABI?

That is the original problem, as you described in #7264 (comment), this will be ignored. We need to check for the real thing.

[SSE4]

oh, I see, you're passing -std=c++11 below

[uilianries]

I can't see the warning message when running it on CentOS6 + Devtools7:

python conans/conan.py profile new default --detect
CC and CXX: /opt/rh/devtoolset-7/root/usr/bin/gcc, /opt/rh/devtoolset-7/root/usr/bin/g++ 
Found gcc 7.3
gcc>=5, using the major as version
Profile created with detected settings: /home/conan/.conan/profiles/default
[conan@eb84f8fab0db project]$ python conans/conan.py profile show default
Configuration for profile default:

[settings]
os=Linux
os_build=Linux
arch=x86_64
arch_build=x86_64
compiler=gcc
compiler.version=7
compiler.libcxx=libstdc++
build_type=Release
[options]
[build_requires]
[env]

For regular GCC7 docker image, based on Ubuntu, it works fine:

$ python conans/conan.py profile new default --detect 
CC and CXX: /usr/bin/gcc, /usr/bin/g++ 
Found gcc 7.2
gcc>=5, using the major as version
Profile created with detected settings: /home/conan/.conan/profiles/default
conan@f5cb86db1e8f:~/project$ python conans/conan.py profile show default          
Configuration for profile default:

[settings]
os=Linux
os_build=Linux
arch=x86_64
arch_build=x86_64
compiler=gcc
compiler.version=7
compiler.libcxx=libstdc++11
build_type=Release
[options]
[build_requires]
[env]

Steps to reproduce:

$ cd <conan_project_folder>
$ docker run --rm -ti -v ${PWD}:/home/conan/project conanio/gcc7-centos6:latest
$ cd project
$ export CONAN_V2_MODE=1
$ export PYTHONPATH=${PWD}
$ python conans/conan.py profile new default --detect
$ python conans/conan.py profile show default

[memsharded]

I can't see the warning message when running it on CentOS6 + Devtools7:

This will not warn if it checks the libcxx correctly, it will just assign the correct value, and seems to work ok, doesn't it?

[uilianries]

This will not warn if it checks the libcxx correctly, it will just assign the correct value, and seems to work ok, doesn't it?

Hmm, correct!