Convert snprint_eq to a variadic macro for MSVC

This fixes a warning emitted by MSVC, because `snprint_eq` is being called with 6 arguments, and the macro expects 5. Variadic macros have been supported in MSVC [since Visual Studio 2005][1].

Discovered while working on https://github.com/libgit2/libgit2/pull/1767.

[1]: http://msdn.microsoft.com/en-us/library/ms177415(v=vs.80).aspx