pgsql: If there is no sigdelset(), define it as a macro.

Log Message:
-----------
If there is no sigdelset(), define it as a macro.

This removes some duplicate code that recreated the identical workaround
when the newer signal API is missing.

Modified Files:
--------------
    pgsql/src/backend/postmaster:
        bgwriter.c (r1.63 -> r1.64)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/bgwriter.c?r1=1.63&r2=1.64)
        walwriter.c (r1.8 -> r1.9)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/walwriter.c?r1=1.8&r2=1.9)
    pgsql/src/backend/tcop:
        postgres.c (r1.576 -> r1.577)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/postgres.c?r1=1.576&r2=1.577)
    pgsql/src/include/libpq:
        pqsignal.h (r1.34 -> r1.35)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/libpq/pqsignal.h?r1=1.34&r2=1.35)