I meant to do this a long time ago, but basically it accounts for the new XP_SOLARIS build flag that never made it into the MailNews code. Additionally, it enables a compatibility flag for Solaris 11.4 that allows us to use the three-argument implementation of ctime_r still used by Solaris 11.3 and illumos (which also appears equivalent to the NSLDAPI_CTIME implementation used by libldap internally). Also, the ctime_r function has been added to the time.h header library for a while now, not sure why Mozilla thought we didn't have a ctime_r implementation.
I didn't include this in my initial PR because I assumed this part
was not relevant to UXP. I was asked to submit this while the MailNews rebase
was going on, but I was late with my patch and didn't quite grasp that this
specifically was still needed.
My counterpart at Mozilla, Petr Sumbara, has gotten his version of this