New comment by JeromeMartinez on void-packages repository

https://github.com/void-linux/void-packages/pull/27936#issuecomment-768594473

Comment:
> They should have #define _POSIX_C_SOURCE at the top of the file and before any #include

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.
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).

Checking autoconf, looks like there is a posibility to do:
AC_CHECK_FUNCS([gmtime_r])
in configure.ac
then adding `#ifdef HAVE_GMTIME_R >=1` in the test should work.
If you are able to test it, I am fine for adding it in the upstream source code.