New comment by ericonr on void-packages repository

https://github.com/void-linux/void-packages/pull/27936#issuecomment-768619214

Comment:
> We would like to have a method which is fine for you but at the same time fine for platforms not having gmtime_r (and also as multithread is not always used, if we can avoid a mutex... I am aware that it is a lot of conditions).

It would be pretty complicated (and error prone) to build one version of the library for the MT or not case, so always using `gmtime_r` is the least bad alternative. It also shouldn't use a mutex, since the second argument allows it to simply not use any globals.

> IIUC doing that will make gmtime_r available in your case but will at the same time create issues on platforms not having gmtime_r, something we are actually trying to avoid.

Do you know of any platforms that don't have it, besides Windows? (For which you already have an ifdef block).