<memory_resource> not found on linux ci

[edhebi]

Might be a CI specific problem or not, gotta test. #38 is dependent on this one getting fixed

[edhebi]

This is the 1st run where it breaks: https://github.com/Edhebi/cpp-sdl2/actions/runs/143202036

[edhebi]

Reproduced on an actual linux machine, it seems to be due to gcc being too old. Apparently, <memory_resource> requires gcc>=9
https://godbolt.org/z/_YG6ji

[edhebi]

@Ybalrid Interested by your opinion on this one : should we mandate gcc9, or conditionaly define the memory resource implementation on __has_include(<memory_resource>) ? we don't actually use the memory resource anywhere else.

[edhebi]

fwiw, this'd allow this part to compile on gcc starting from 7.5 at least (the one on the CI and latest on ubuntu repos):
https://godbolt.org/z/vjx5Px

[Ybalrid]

If you want to maintain codepath with and without this header, it's your call not mine