[WEC] building boost for Windows Embedded Compact 2013

I want to use Boost.Dll under Windows Embbedded Compact 2013, but I've to build the Boost.System for WEC to do so.

https://github.com/boostorg/dll/issues/10
https://github.com/boostorg/winapi/issues/38
https://svn.boost.org/trac/boost/ticket/12853#comment:1

Normally I build static boost with the following commands:
```
bootstrap.bat
.\b2.exe headers
.\b2.exe variant=release,debug link=static runtime-link=static threading=multi address-model=32 architecture=x86 --toolset=msvc-15.0 --without-python --without-mpi --stagedir=stage stage
```

The WEC SDK 2013 comes bundled with a Microsoft Toolchain (VS 2012 based) like the Windows 7.1 SDK.
What are correct `--toolset=` settings for the build?