system_error: inline error_category ctor on Win32

[compnerd]

Summary

Because the class is marked as dllexport and instantiated in
algorithm.cpp and is strongly defined in system_error.cpp, we end up
with multiple definitions. By keeping the definition as an inline
definition, the dllexport will instantiate it. Being inlined, they
will be marked as COMDAT(SELECT_ANY) and will still be exported from the
dll.

[EricWF]

I committed a different fix for this patch in r290840.

Instead of just fixing this on Windows it makes the removal of this definition an ABI option, which it then disables on Windows by default.

[compnerd]

WFM, that seems like a nicer option.

src/system_error.cpp

 
 #include "__config"
 
+#if !defined(_WIN32)
 #define _LIBCPP_BUILDING_SYSTEM_ERROR
+#endif
 #include "system_error"
 
 #include "include/config_elast.h"

 
 // class error_category
 
+#if !defined(_WIN32)
 error_category::error_category() _NOEXCEPT
 {
 }
+#endif
 
 error_category::~error_category() _NOEXCEPT
 {