[CMake] Is there a way to obtain the current compiler standard flag?

[Chuck Atkins]

Hi Chris.

Try using the the CMAKE_CXX${std}_STANDARD_COMPILE_OPTION variable.  For
example, the folowing piece of CMake code:
cmake_minimum_required(VERSION 3.9)

project(foo CXX)

foreach(std IN ITEMS 98 11 14 17)
  message("C++${std} std flags: ${CMAKE_CXX${std}_STANDARD_COMPILE_OPTION}")
  message("C++${std} ext flags:
${CMAKE_CXX${std}_EXTENSION_COMPILE_OPTION}")
endforeach()

Wll generate for GCC on Linux:
-- The CXX compiler identification is GNU 8.1.1
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
C++98 std flags: -std=c++98
C++98 ext flags: -std=gnu++98
C++11 std flags: -std=c++11
C++11 ext flags: -std=gnu++11
C++14 std flags: -std=c++14
C++14 ext flags: -std=gnu++14
C++17 std flags: -std=c++1z
C++17 ext flags: -std=gnu++1z
-- Configuring done

PGI on Linux:
-- The CXX compiler identification is PGI 18.4.0
-- Check for working CXX compiler: /opt/pgi/linux86-64/18.4/bin/pgc++
-- Check for working CXX compiler: /opt/pgi/linux86-64/18.4/bin/pgc++ --
works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
C++98 std flags: -A
C++98 ext flags: --gnu_extensions
C++11 std flags: --c++11;-A
C++11 ext flags: --c++11;--gnu_extensions
C++14 std flags: --c++14;-A
C++14 ext flags: --c++14;--gnu_extensions
C++17 std flags: --c++17;-A
C++17 ext flags: --c++17;--gnu_extensions
-- Configuring done


And for IBM XL on AIX:
-- The CXX compiler identification is XL 13.1.3
-- Check for working CXX compiler: /usr/bin/xlC
-- Check for working CXX compiler: /usr/bin/xlC -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
C++98 std flags: -qlanglvl=strict98
C++98 ext flags: -qlanglvl=extended
C++11 std flags: -qlanglvl=extended0x
C++11 ext flags: -qlanglvl=extended0x
C++14 std flags:
C++14 ext flags:
C++17 std flags:
C++17 ext flags:
-- Configuring done
-- Generating done

- Chuck
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