[locale] Loss of precision (C4244) warnings in MSVC 64-bit build

From: Kenneth Porter (shiva_at_[hidden])
Date: 2019-07-24 05:08:34

Building with MSVC compilers for 64-bit results in several C4244 warnings 
in api.hpp, due to passing the difference of two pointers (ptrdiff_t) to 
Win APIs expecting an int.

Patch here:

<https://github.com/boostorg/locale/issues/42>

Using this command with the recent 1.71.0 beta:

call .\bootstrap.bat --without-python

.\b2 --without-python toolset=msvc-14.2,msvc-14.0,msvc-12.0,msvc-10.0 
runtime-link=shared,static address-model=32,64