pgsql: In a Windows backend, don't build src/port/pgsleep.c's version of

Log Message:
-----------
In a Windows backend, don't build src/port/pgsleep.c's version of
pg_usleep at all.  Instead call the replacement function in
port/win32/signal.c by that name.  Avoids tricky macro-redefinition
logic and suppresses a compiler warning; furthermore it ensures that
no one can accidentally use the non-signal-aware version of pg_usleep
in a Windows backend.

Modified Files:
--------------
    pgsql/src/backend/port/win32:
        signal.c (r1.16 -> r1.17)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/port/win32/signal.c.diff?r1=1.16&r2=1.17)
    pgsql/src/backend/postmaster:
        syslogger.c (r1.27 -> r1.28)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/syslogger.c.diff?r1=1.27&r2=1.28)
    pgsql/src/include/port:
        win32.h (r1.52 -> r1.53)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/port/win32.h.diff?r1=1.52&r2=1.53)
    pgsql/src/port:
        pgsleep.c (r1.7 -> r1.8)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/port/pgsleep.c.diff?r1=1.7&r2=1.8)