pgsql: Add support to port/snprintf.c for position parameter

Log Message:
-----------
Add support to port/snprintf.c for position parameter specification:

+ # Determine if printf supports %1$ argument selection, e.g. %5$ selects
+ # the fifth argument after the printf print string.
+ # This is not in the C99 standard, but in the Single Unix Specification (SUS).
+ # It is used in our langauge translation strings.

Nicolai Tufar with configure changes by Bruce.

Modified Files:
--------------
    pgsql:
        configure (r1.425 -> r1.426)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/configure.diff?r1=1.425&r2=1.426)
        configure.in (r1.399 -> r1.400)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/configure.in.diff?r1=1.399&r2=1.400)
    pgsql/config:
        c-library.m4 (r1.29 -> r1.30)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/config/c-library.m4.diff?r1=1.29&r2=1.30)
    pgsql/src/port:
        snprintf.c (r1.4 -> r1.5)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/port/snprintf.c.diff?r1=1.4&r2=1.5)