make libconfig exceptions public

it's allow to generate libconfig exception in user code
example:
libconfig::Setting& obj = ...;
if ( !obj.isGroup() )
    throw libconfig::SettingTypeException( obj, "TypeGroup expected" );

lib/libconfig.h++

@@ -54,8 +54,6 @@ class Setting; // fwd decl
 
 class LIBCONFIGXX_API SettingException : public ConfigException
 {
-  friend class Config;
-
   public:
 
   SettingException(const SettingException &other);
@@ -67,8 +65,6 @@ class LIBCONFIGXX_API SettingException : public ConfigException
 
   virtual const char *what() const throw();
 
-  protected:
-
   SettingException(const Setting &setting);
   SettingException(const Setting &setting, int idx);
   SettingException(const Setting &setting, const char *name);
@@ -81,15 +77,10 @@ class LIBCONFIGXX_API SettingException : public ConfigException
 
 class LIBCONFIGXX_API SettingTypeException : public SettingException
 {
-  friend class Config;
-  friend class Setting;
-
   public:
 
   virtual const char *what() const throw();
 
-  private:
-
   SettingTypeException(const Setting &setting);
   SettingTypeException(const Setting &setting, int idx);
   SettingTypeException(const Setting &setting, const char *name);
@@ -97,15 +88,10 @@ class LIBCONFIGXX_API SettingTypeException : public SettingException
 
 class LIBCONFIGXX_API SettingNotFoundException : public SettingException
 {
-  friend class Config;
-  friend class Setting;
-
   public:
 
   virtual const char *what() const throw();
 
-  private:
-
   SettingNotFoundException(const Setting &setting, int idx);
   SettingNotFoundException(const Setting &setting, const char *name);
   SettingNotFoundException(const char *path);
@@ -113,15 +99,10 @@ class LIBCONFIGXX_API SettingNotFoundException : public SettingException
 
 class LIBCONFIGXX_API SettingNameException : public SettingException
 {
-  friend class Config;
-  friend class Setting;
-
   public:
 
   virtual const char *what() const throw();
 
-  private:
-
   SettingNameException(const Setting &setting, const char *name);
 };
 
@@ -134,8 +115,6 @@ class LIBCONFIGXX_API FileIOException : public ConfigException
 
 class LIBCONFIGXX_API ParseException : public ConfigException
 {
-  friend class Config;
-
   public:
 
   ParseException(const ParseException &other);
@@ -153,10 +132,10 @@ class LIBCONFIGXX_API ParseException : public ConfigException
 
   virtual const char *what() const throw();
 
-  private:
-
   ParseException(const char *file, int line, const char *error);
 
+  private:
+
   const char *_file;
   int _line;
   const char *_error;