Fix #644. Export specializations of BasicError

- Only export BasicError::setMsg which is the only implementation present in the .cpp
- Only export BasicError specialization on __APPLE__

It seems that gcc automatically adds the attribute when the visibility
settings are set to hidden. See this link for more information:
https://reviews.llvm.org/D35388

cmake/mainSetup.cmake

@@ -13,8 +13,10 @@ set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)
 set(CMAKE_RUNTIME_OUTPUT_DIRECTORY_DEBUG ${CMAKE_RUNTIME_OUTPUT_DIRECTORY})
 set(CMAKE_RUNTIME_OUTPUT_DIRECTORY_RELEASE ${CMAKE_RUNTIME_OUTPUT_DIRECTORY})
 
-set(CMAKE_CXX_VISIBILITY_PRESET hidden)
-set(CMAKE_VISIBILITY_INLINES_HIDDEN 1)
+if (NOT CMAKE_CXX_COMPILER_ID STREQUAL "AppleClang")
+    set(CMAKE_CXX_VISIBILITY_PRESET hidden)
+    set(CMAKE_VISIBILITY_INLINES_HIDDEN 1)
+endif()
 
 set(CMAKE_CXX_STANDARD 98)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)

include/exiv2/error.hpp

@@ -315,7 +315,7 @@ namespace Exiv2 {
         std::basic_string<charT> arg3_;         //!< Third argument
         std::string              msg_;          //!< Complete error message
 #ifdef EXV_UNICODE_PATH
-    std::wstring             wmsg_;         //!< Complete error message as a wide string
+        std::wstring             wmsg_;         //!< Complete error message as a wide string
 #endif
     }; // class BasicError
 
@@ -389,6 +389,7 @@ namespace Exiv2 {
         return wmsg_.c_str();
     }
 #endif
+
 #ifdef _MSC_VER
 # pragma warning( default : 4275 )
 #endif

src/error.cpp

@@ -224,7 +224,6 @@ namespace Exiv2 {
     {
     }
 
-    //! @cond IGNORE
     template<>
     void BasicError<char>::setMsg()
     {
@@ -257,7 +256,9 @@ namespace Exiv2 {
         wmsg_ = s2ws(msg);
 #endif
     }
-    //! @endcond
+#ifdef __APPLE__
+    template class EXIV2API BasicError<char>;
+#endif
 
 #ifdef EXV_UNICODE_PATH
     template<>
@@ -291,6 +292,7 @@ namespace Exiv2 {
         wmsg_ = wmsg;
         msg_ = ws2s(wmsg);
     }
+    template class EXIV2API BasicError<wchar_t>;
 #endif
 
     const char* errMsg(int code)