[boost] [xpressive] Merge PR for fixing MSVC 14.1

Subject: [boost] [xpressive] Merge PR for fixing MSVC 14.1
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2017-06-30 21:39:23

Hi,

There is this PR from Daniela Engert to fix Boost.Xpressive compilation 
failure with MSVC 14.1:

https://github.com/boostorg/xpressive/pull/4

It replaces the removed std::unary_function with the equivalent 
typedefs. The change looks rather harmless.

Could this PR be merged before 1.65 is out?

Thanks.