Set WINVER to WIN7 for libc++

This is needed to prevent libc++ from adding a dependency on
GetSystemTimePreciseAsFileTime, which was introduced in Windows 8.

See the thread with subject "should _WIN32_WINNT be set to _WIN32_WINNT_WIN10 or
_WIN32_WINNT_WIN7" [1] on chromium-dev for more information.

[1] https://groups.google.com/a/chromium.org/forum/#!topic/chromium-dev/QP4cZtF7B0E

BUG=801780
R=thakis

Change-Id: Idd10dc296d383cbb0fb25d2e95d6b4c2f08ac0c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1524992
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Auto-Submit: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#641355}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: fdf6331fb04b1bdbed52758c8008babbefcbaabb
1 file changed