Baseline bump to MSVC 2015 Update 3

[Caolán McNamara]

On Thu, 2017-03-02 at 17:02 +0100, Stephan Bergmann wrote:
> As has been brewing for some time and just got accepted in today's
> ESC meeting, we'll bump master (towards LO 5.4) to MSVC 2015 Update 3
> as basline for MSVC builds on Windows.  That will happen as soon as 
> <https://gerrit.libreoffice.org/#/c/22588/> "Drop support for MSVC
> 2013" builds green and is pushed.

That means that _MSC_VER is always >= 1900 (?), I presume, outside of
the headers exposed through the sdk, that means we can remove the ifdef
branches that support earlier versions of that now ?