mingw: require Windows Vista or later (and fix the Windows CI builds)

I noticed that a recent GitGitGadget build failed in the Windows phase, with compat/poll/poll.h problems all over the place.

Turns out that this is caused by the recent upgrade of the mingw-w64 headers and crt files (7.0.0.5233.e0c09544 -> 7.0.0.5245.edf66197, which I assume now enforces Vista as minimal Windows version also for all mingw-w64 projects).

Luckily, in Git for Windows' master, we already had changes to require Vista (for unrelated reasons: to restrict the std handle inheritance when spawning new processes).

Technically, Windows Vista is already no longer supported
[https://support.microsoft.com/en-us/help/22882/windows-vista-end-of-support]
, but we do try to keep Git building on older Windows version, up until the point when it becomes too big of a maintenance burden.

Johannes Schindelin (3):
compat/poll: prepare for targeting Windows Vista
mingw: set _WIN32_WINNT explicitly for Git for Windows
mingw: bump the minimum Windows version to Vista

 compat/poll/poll.c |  6 +++---
 compat/poll/poll.h | 15 +++++++++++++++
 config.mak.uname   |  4 ----
 git-compat-util.h  |  4 ++--
 4 files changed, 20 insertions(+), 9 deletions(-)

base-commit: fe8321ec057f9231c26c29b364721568e58040f7
Published-As: https://github.com/gitgitgadget/git/releases/tags/pr-44%2Fdscho%2Frequire-windows-vista-or-later-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-44/dscho/require-windows-vista-or-later-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/44

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