Enable MSVC conforming preprocessor

Enable MSVC conforming preprocessor

Switch MSVC to use the conforming preprocessor, using the
/Zc:preprocessor option.

This allows us to drop the alternative implementation of
VA_ARGS_NARGS() for the previous "traditional" preprocessor.

This also prepares the way for enabling C11 mode in the future, which
enables the conforming preprocessor by default.

This now requires Visual Studio 2019.  The installation documentation
is adjusted accordingly.

Discussion: https://www.postgresql.org/message-id/flat/01a69441-af54-4822-891b-ca28e05b215a%40eisentraut.org