symbol visibility problem on macOS

When I compile with gcc-9, gcc-10, clang-9, clang-10 or the Xcode clang I have this problem. I do not see it under Linux (Ubuntu 18 LTS):
[BUG-REPORT.txt](https://github.com/boostorg/program_options/files/4944925/BUG-REPORT.txt)

I get linkage warnings that suggest possible runtime failure

ld: warning: direct access in function '__ZN5boost10wrapexceptINS_15program_options14unknown_optionEEC1ERKS3_' from file lib/libboost_program_options.a(options_description.o)' to global weak symbol '__ZTVN5boost15program_options5errorE' from file XXX means the weak symbol cannot be overridden at runtime. This was likely caused by different translation units being compiled with different visibility settings.

What's weird is that visibility should only apply to dynamic linkage, and I have built the libraries entirely statically.

I was able to get around this by building all of boost by supplying `cxxflags="-fvisibility=default"` to b2. Probably not the right fix but make the problem go away.

Probably incomplete list of symbols causing this problem. I do not have this issue with any other Boost library I use.

Beginne mit BUG-REPORT.txt und reproduziere den statischen Build unter macOS mit den aufgeführten GCC-, Clang- oder Xcode-Clang-Versionen. Vergleiche den Build mit und ohne b2's cxxflags="-fvisibility=default" und prüfe die Linker-Warnungen zu program_options-Symbolen. Als erledigt gilt die Aufgabe, wenn die Visibility-Warnungen ohne Abhängigkeit von diesem Workaround behoben sind.

Vom Indexierungsmodell aus dem Issue-Text verfasst.