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

Reproduce the linker failure with the linked BDAMinimalTestCase using CMake and MSVC Build Tools 2015 or 2017. Start with the verbose linker log and the exports from program_options.dll, then determine whether the two unresolved symbols are a supported library issue; done means a confirmed cause and a documented or validated resolution.

Scritto dal modello di indicizzazione a partire dal testo della issue.