... should be more platforms ... but be conservative.
U_PLATFORM_IMPLEMENTS_POSIX does not hold what it promises.

"The file and this data structure is not standardized.  Don't rely on it.  It
can go away without warning."

...

And since glibc 2.26 it's gone.

https://ssl.icu-project.org/trac/ticket/13329

Change-Id: I4c1f6130571f5d094cb35ce70e4d333763cee32a
(cherry picked from commit 9e399bc6)
Reviewed-on: https://gerrit.libreoffice.org/46584Reviewed-by: Jan Holesovsky <kendy@collabora.com>
Tested-by: Jan Holesovsky <kendy@collabora.com>