Fix export/import of DLL symbols

[SpaceIm]

• Remove CMAKE_WINDOWS_EXPORT_ALL_SYMBOLS, which is unnecessary when symbols are properly exported
• Do not define REPLXX_BUILDING_DLL at consume time. It must be PRIVATE, you don't want dllexport at consume time, only dllimport
• Add missing REPLXX_IMPEXP in front of several functions in public headers.