C++: Improve error message when finding X11 macro Unsorted

[emilk]

• Closes enum SortingStatus::Unsorted conflicts with define in X.h #7846

Some C library uses macros for constants, which makes those identifiers unusable by any other library in the same global namespace. So anyone including both X.h and rerun.hpp in the same compilation unit (in that order) would have problems. It's a dumpster-fire.

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[rgolovanov]

@emilk , @Wumpf
We probably shouldn't do it inside the rerun's header. I would leave it to the end user to decide how to deal with this. Maybe, maximum, what rerun could do is to show #pragma warning explaining the case to the user and the user would need to do undef/define properly.

In current solution, if you #undef Unsorted then it may potentially break something else for the user...

[emilk]

Something like

#ifdef Unsorted
#    error "There is a conflicting macro 'Unsorted' (probably from X11). Either include <rerun.hpp> FIRST, OR add '#undef Unsorted' before '#include <rerun.hpp>"
#endif

?

[Wumpf]

I think I have a slight preference for the error message, as rgolovanov says otherwise we might break a user unexpectedly.

Either include <rerun.hpp> FIRST,

that part is not a good suggestion because it means that if you only do that you'll still break when trying to use rerun::SortingStatus::Unsorted. Yes, it will compile as long as you don't use it, but it just breaks later then in your code

I'd suggest:

#ifdef Unsorted
#    error "There is a macro 'Unsorted' (probably from X11) defined, incorrectly replacing one of the variants of `rerun::SortingStatus`. Add '#undef Unsorted' before '#include <rerun.hpp>' to work around this."
#endif

that then essentially only assumes that the user will know what happens if they get an error for Unsorted not being known to the compiler down the line (which is fairly unlikely, but who knows!)