MSVC: Turn missing function declaration into an error

MSVC: Turn missing function declaration into an error

Calling an undeclared function should be an error as of C99, and GCC
and Clang do that, but MSVC doesn't even warn about it in the default
warning level.  (Commit c86d2ccdb35 fixed an instance of this
problem.)  This turns on this warning and makes it an error by
default, to match other compilers.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/1142ad86-e475-41b3-aeee-c6ad913064fa%40eisentraut.org