There are few instances where OpenSSL requires a POSIX C library, at least version 1-2008, or compatible enough functionality.
There are exceptions, though, for platforms that do not have a POSIX
library, or where there are quirks that need working around. A notable
platform is Windows, where POSIX functionality may be available, but where
the function names are prefixed with an underscore, and where some POSIX
types are not present (such as ssize_t).
Platforms that do have a POSIX library may still not have them accessible unless the following macros are defined:
_POSIX_C_SOURCE=200809L
_XOPEN_SOURCE=1
This is, for example, the case when building with gcc or clang and using the
flag -ansi.