linking with program_options has unresolved symbols on MSVC

I'm developing a simple command line client application against boost::program_options. Everything works fine on Linux with gcc-4.8, gcc-5.3 and gcc-6.3, on Darwin with XCode 7 and on Windows with MinGW-w64. But on Windows with MSVC Build Tools x64 from 2015 and 2017 I get two unresolved symbols:

```
LightBISClientCMDLine.cc.obj : error LNK2001: unresolved external symbol "class std::basic_string,class std::allocator > boost::program_options::arg" (?arg@program_options@boost@@3V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@A)
LightBISClientCMDLine.cc.obj : error LNK2001: unresolved external symbol "public: static unsigned int const boost::program_options::options_description::m_default_line_length" (?m_default_line_length@options_description@program_options@boost@@2IB)
```

I checked program_options.dll and I am under the impression that those two symbols are defined there. I get exact matches for `?arg@program_options@boost@@3V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@A` and ?m_default_line_length@options_description@program_options@boost@@2IB.` Also all other symbols from program_options are resolved correctly. Here is the details build log with a verbose linker message: ​http://data.biodataanalysis.de/tmp/boost_program_options_linker_error_emmenlau.txt

I checked the build log to see that the library was built with `-DBOOST_PROGRAM_OPTIONS_DYN_LINK=1`.

As far as I could see, it's not supported to export STL classes like `std::string` from a DLL with Visual Studio 2015 and 2017 because it seems considered bad practice. There are various reports that indicate that exporting STL classes can cause issues when mixing different runtimes or compiler versions. Is it possible that Microsoft just disabled that option alltogether?

I found related reports here:
* ​https://social.msdn.microsoft.com/Forums/vstudio/en-US/df99b712-c00b-4af0-82fd-3764c8b6cbec/exporting-stdstring-from-a-dll-does-not-export-stdstringnpos
* ​https://stackoverflow.com/a/13866797/7200132

Here is an example that shows the error. The build is configured with cmake: https://github.com/emmenlau/BDAMinimalTestCase

Reproduzieren Sie den Linker-Fehler mit dem verknüpften BDAMinimalTestCase unter Verwendung von CMake und MSVC Build Tools 2015 oder 2017. Beginnen Sie mit dem ausführlichen Linker-Protokoll und den Exporten aus program_options.dll und ermitteln Sie anschließend, ob die beiden nicht aufgelösten Symbole auf ein unterstütztes Bibliotheksproblem zurückzuführen sind; abgeschlossen ist die Aufgabe, wenn eine bestätigte Ursache sowie eine dokumentierte oder validierte Lösung vorliegen.

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