msvc const warnings

MSVC generates a number of const warnings, that's been discussed before
(http://archives.postgresql.org/pgsql-hackers/2007-01/msg01214.php).

Attached patch adds a pragma to get rid of the warnings. Again, not sure if
it's worth it?

I think I need to #ifdef the pragma - gcc generates a warning if it sees it
in -Wall, and that'd certainly be worse :-)

Stefan mentioned that the warning may be one that shows up in a different
compiler somewhere as well, thouh, which might indicate that we should fix
the underlying issue? (Even if the code is correct, if it confuses multiple
compilers...)

//Magnus