Motivation / Problem

Compilation with MinGW fails since #7441 has been merged.
After some digging in headers, I found MinGW defaults to Windows 7 target (and cause some unexpected stuff, like GF_END to be defined), while we set MSVC to target Windows XP.

Description

Set the same target for all Windows compilers.
I needed to move the definitions on top of stdafx.h because any MinGW header (even non windows API ones) will define _WIN32_WINNT 0x0601 if it's not yet defined.
I also needed to reorder #include lines in some files for the same reason, stdafx.h MUST be the first included file :)

Limitations

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, gs_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker