Repleacement for src/port/snprintf.c

Hello all,

I would like to submit my changes to src/port/snprintf.c to
enable %n$ format placeholder replacement in snprintf() and
vsnprintf(). Additionally I implemented a trivial printf().

I also attach a diff for configure.in to include snprintf.o
in pgport but I am sure it is not the right thing to do.
Could someone give a hint on where I need to place such a
definition.

Please review my patch. as Tom Lane pointed out there
are 150 messages in the following files that do not print
properly:

src/backend/po/pt_BR.po
src/backend/po/de.po
src/backend/po/es.po
src/backend/po/zh_CN.po
src/backend/po/tr.po
src/bin/pg_dump/po/zh_CN.po
src/bin/pg_dump/po/tr.po
src/bin/psql/po/zh_CN.po
src/bin/psql/po/zh_TW.po
src/bin/psql/po/tr.po
src/bin/scripts/po/zh_CN.po

we need to fix snprintf.c before next release

Best regards,
Nicolai Tufar