Define _POSIX_C_SOURCE as 200112L on Solaris.

Define _POSIX_C_SOURCE as 200112L on Solaris.

This is an attempt to suppress some compiler warnings that appeared in
the wake of commit 7f798aca1: it seems that by default Solaris/illumos
declares shmdt() to take "char *" not "void *".  We'd like the system
headers to provide modern POSIX APIs, and POSIX 2001 seems to be as
modern as is available there.

illumos' standards(7) man page suggests that we might also need to
define __EXTENSIONS__, but let's see what happens with just this.

Discussion: https://postgr.es/m/1654508.1733162761@sss.pgh.pa.us