Minimum version of Windows for default branch

[Markus Mützel]

For the Windows builds, Octave currently defines its minimum Windows API level 
to Windows NT 4.0 with SP3 (_WIN32_WINNT is set to 0x0403). 
That switch is used to define the minimum version of the Windows API a program 
is expecting. Changing this value does not immediately prohibit starting a 
program on older Windows systems. But it allows to use API functions that are 
only available in newer versions.

Since Qt dropped support for Windows after version Qt 5.6.3, it is no longer 
possible to use the GUI with the official installer in Windows XP. Also mesa 
seems to meanwhile have dropped support for Windows XP. But it seems to still 
be possible to run Octave CLI when using an older opengl32.dll (or without the 
bundled OpenGL software rendering driver?, see bug #54662 [1]).

For bug #36477 [2], we might want to use API functions that require Windows 
Vista or newer. That change would prevent users from executing Octave on 
Windows XP.

We could probably still use older (deprecated) Windows API functions until they 
will finally be removed in a future Windows version. But in the light of more 
and more dependencies dropping support for Windows XP, would it be OK to 
require Windows Vista or newer?

Markus

[1]: https://savannah.gnu.org/bugs/?54662
[2]: https://savannah.gnu.org/bugs/?36477