[build2] build2 0.6.2 bootstrapping broken with update 5 of VS 2017

[Gerardo Hernandez]

The build2 bootstrapping is broken cause _MSC_VER has increased to 1912 in
Visual Studio 2017 update 5, causing errors like:

C:\Program Files (x86)\Microsoft Visual
Studio\2017\Community\VC\Tools\MSVC\14.12.25827\include\regex(5061): error
C2039: '_Ch_upper': is not a member of
'std::regex_traits<build2::test::script::regex::line_char>' (compiling
source file regex.cxx)
during bootstrapping.

I have attached the patch to solve it, but unfortunately I reckon some
packages in cppget.org will need updating as well. E.g. the error

c++ bpkg-0.6.0\bpkg\cxx{bpkg}
C:\projects\build2-toolchain-0.6\build2-0.6.1\build2/function.hxx(123):
error C2338: type is not POD

shows up after fetching and trying to compile some packages. It would be
solved in the same way the patch does.
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