Include directories should be marked as system includes

[hadrielk]

When compiling fmt by the direct CMake add_subdirectory(fmt) method (as recommended in the docs), the fmt build inherits the same compilation flags as the project including it.

But it fails for various errors/warnings, in clang at least.

For example missing-noreturn, signed-enum-bitfield, and documentation-unknown-command:

In file included from /fmt/include/fmt/format.h:48:
/fmt/include/fmt/core.h:451:5: error: unknown command tag name [-Werror,-Wdocumentation-unknown-command]
    \rst
    ^~~~
/fmt/include/fmt/core.h:604:38: error: function 'on_error' could be declared with attribute 'noreturn' [-Werror,-Wmissing-noreturn]
  void on_error(const char* message) { throw_format_error(message); }
                                     ^
/fmt/include/fmt/core.h:2075:9: error: enums in the Microsoft ABI are signed integers by default; consider giving the enum type an unsigned underlying type to make this code portable [-Werror,-Wsigned-enum-bitfield]
        align(align::none),
        ^

There were more instances than the above, but this gives the idea.

From my perspective, users of fmt would normally consider it a system include, not a regular part of their codebase. So I suggest that for the target_include_directories() in fmt/CMakeLists.txt, the SYSTEM cmake attribute be used, or at least a CMake OPTION be available to do so.

[vitaut]

This should definitely not be the default because it would just hide potentially useful diagnostic but a PR to add a CMake option would be welcome.

[daquexian]

FYI, CMake 3.25 introduces add_subdirectory(... SYSTEM) to mark 3rd libraries as system includes: https://cmake.org/cmake/help/latest/command/add_subdirectory.html