socketpair() is not in libc on illumos

On illumos, the socketpair() function is in libsocket and not libc.

https://illumos.org/man/socketpair

Running the testsuite in version 1.14.0 results in:

#5  0xfffffc7fef2d7cac in _dbus_warn (
    format=0xfffffc7fef2e87b0 "_dbus_socketpair() not implemented on this OS")
    at dbus-internals.c:257

The configure test for socketpair() should probably be implemented with

AC_SEARCH_LIBS(socketpair,[socket])

as is already done for the socket() function.