Fix wxString::GetCache() compilation in UTF-8 DLL build with MSVS

Don't declare static Cache variable inside wxString declaration itself
because it is implicitly DLL-exported, due to the use of
__declspec(dllexport) for the entire wxString class, but thread-specific
variables can't be exported, so this resulted in a compilation error.

Avoid this by using a static thread-specific variable inside GetCache(),
which had to be moved out of line.

include/wx/string.h

@@ -447,8 +447,8 @@ private:
       unsigned lastUsed;
   };
 
-  static wxTHREAD_SPECIFIC_DECL Cache ms_cache;
-  static Cache& GetCache() { return ms_cache; }
+  // Implemented out of line because per-thread variable can't be DLL exported.
+  static Cache& GetCache();
 
   static Cache::Element *GetCacheBegin() { return GetCache().cached; }
   static Cache::Element *GetCacheEnd() { return GetCacheBegin() + Cache::SIZE; }

src/common/string.cpp

@@ -75,7 +75,12 @@ const wxStringCharType WXDLLIMPEXP_BASE *wxEmptyStringImpl = "";
 const wxChar WXDLLIMPEXP_BASE *wxEmptyString = wxT("");
 #if wxUSE_STRING_POS_CACHE
 
-wxTHREAD_SPECIFIC_DECL wxString::Cache wxString::ms_cache;
+/* static */
+wxString::Cache& wxString::GetCache()
+{
+    static wxTHREAD_SPECIFIC_DECL Cache s_cache;
+    return s_cache;
+}
 
 // gdb seems to be unable to display thread-local variables correctly, at least
 // not my 6.4.98 version under amd64, so provide this debugging helper to do it