Prohibit manually built paths that point in srcdir in include_directo…

[jpakkane]

…ries and give information on what to use instead.

[nirbheek]

Should also mention here that you can use any relative path to any subdirectory otherwise people will think they have to create meson.build files in directories that only contain headers just to get that value set.

[ePirat]

@jpakkane This came around to bite me in VLC, where I pass a path that I get from a cross/native file which can end up pointing in the source dir (depending where users decide to do things) and would error out…

Is there any reason meson can not just make the path relative itself, maybe guarded behind an allow_absolute_to_srcdir kwarg or so? I can not even easily work around this as #7908 is not merged yet, but even if it were it still feels a bit like meson is second-guessing me here.

Or, just do the check in include_directories() which is frequently used to refer to relative directories in the source tree, but not do it when a string is given directly.

[dcbaker]

Why do you need to pass include_paths in the native/cross file? Is it because you need a dependency that meson can't find?

[ePirat]

@dcbaker It's just so that I can cover some edge cases where I need to search for headers in the include dir and cover some cases that need the proper include path for our prebuilt deps without having any explicit dependency on anything in there.