RFR: JDK-8285730: unify _WIN32_WINNT settings

[Erik Joelsson]

On Wed, 27 Apr 2022 14:57:41 GMT, Matthias Baesken <mbaesken at openjdk.org> wrote:

> Currently we set _WIN32_WINNT at various places in the codebase; this is used to target a minimum Windows version we want to support. See also for more detailled information :
> https://docs.microsoft.com/en-us/windows/win32/winprog/using-the-windows-headers?redirectedfrom=MSDN#setting-winver-or-_win32_winnt
> Macros for Conditional Declarations
> "Certain functions that depend on a particular version of Windows are declared using conditional code. This enables you to use the compiler to detect whether your application uses functions that are not supported on its target version(s) of Windows."
> 
> However currently we have quite a lot of differing settings of _WIN32_WINNT in the codebase ; setting _WIN32_WINNT to 0x0601 (Windows 7) where possible would make sense because we have this setting already at   java_props_md.c  (so targeting older Windows versions at other places is most likely not useful).

Requiring different windows versions in different parts of the product doesn't make much sense, but I think it's even worse trying to keep all these different locations in sync. At least most of these have a comment explaining why that particular Windows version is required there. Changing these values invalidates those comments. 

If we are to do something about this, then we need to define this macro in a central location, and verify the value in configure. Then we would provide it either on compile command lines, or in a globally included config.h.

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PR: https://git.openjdk.java.net/jdk/pull/8428