Merge branch 'backport_gcc9' into 'v2.1-stable'

# TODO: at some point this should probably be replaced by a proper use of CMAKE_CXX_COMPILE_FEATURES

# https://cmake.org/cmake/help/latest/manual/cmake-compile-features.7.html

# Minimum GCC versions for C++14 and C++17 feature support.

# Minimum GCC versions for C++17 feature support.

# based on

# https://gcc.gnu.org/projects/cxx-status.html

# https://gcc.gnu.org/onlinedocs/libstdc++/manual/status.html#status.iso.2017

SET(GCC_CXX_14_VERSION_MIN "6.1")

SET(GCC_CXX_17_VERSION_MIN "8.0")

# Note: Version >=9 required by MagicEnum

SET(GCC_VERSION_MIN "9.0")

# Minimum Clang versions for C++14 and C++17 feature support.

# Minimum Clang versions for C++17 feature support.

# based on

# https://clang.llvm.org/cxx_status.html

SET(CLANG_CXX_14_VERSION_MIN "3.4")

SET(CLANG_CXX_17_VERSION_MIN "7.0")

SET(CLANG_VERSION_MIN "7.0")

# Minimum Apple Clang versions for C++14 and C++17 feature support.

# Minimum Apple Clang versions for C++17 feature support.

# based on

# https://trac.macports.org/wiki/XcodeVersionInfo

SET(APPLECLANG_CXX_14_VERSION_MIN "6.1")

SET(APPLECLANG_CXX_17_VERSION_MIN "10.0")

IF(CMAKE_CXX_STANDARD EQUAL 17)

    SET(GCC_VERSION_MIN ${GCC_CXX_17_VERSION_MIN})

    SET(CLANG_VERSION_MIN ${CLANG_CXX_17_VERSION_MIN})

    SET(APPLECLANG_VERSION_MIN ${APPLECLANG_CXX_17_VERSION_MIN})

ELSE()

    SET(GCC_VERSION_MIN ${GCC_CXX_14_VERSION_MIN})

    SET(CLANG_VERSION_MIN ${CLANG_CXX_14_VERSION_MIN})

    SET(APPLECLANG_VERSION_MIN ${APPLECLANG_CXX_14_VERSION_MIN})

ENDIF()

SET(APPLECLANG_VERSION_MIN "10.0")

IF(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")

    IF(CMAKE_CXX_COMPILER_VERSION VERSION_LESS GCC_VERSION_MIN)