Return to using a named constant

[irwir]

The commit Use WINVER values instead of WIN32_WINNT[NAME]. goes against common practice of avoiding numerical constants.
It might be better to add conditional directives into one of the header files:

#if !defined(_WIN32_WINNT_VISTA)
#define _WIN32_WINNT_VISTA` 0x0600
#endif

Optionally, the value could be verified for being 0x0600 in #else branch.

With this change code might return to short and clear version as it was before Fix compilation when _WIN32_WINNT_VISTA macro is not defined

[miniupnp]

@irwir what do you mean ?
The commit 23f492f does not uses a numerical constant.

I guess you are talking about c1b6474

[irwir]

@miniupnp, you are right. I amended the issue text above.