Add wxUSE_PRIVATE_FONTS and drop wxHAS_PRIVATE_FONTS

Handle this feature as all the other ones and provide a configure switch
and a setup.h option to disable it if necessary, as it may be desirable
to do it, especially under Linux, to avoid extra dependency on pangoft2
if this functionality is unnecessary.

src/common/fontcmn.cpp

@@ -1136,16 +1136,3 @@ bool wxFromString(const wxString& str, wxFontBase *font)
 
     return font->SetNativeFontInfo(str);
 }
-
-#ifndef wxHAS_PRIVATE_FONTS
-
-// Adding private fonts is not supported on this platform, so provide the
-// functions that would be used, but make them no-ops that return a code
-// that indicates failure.
-
-bool wxFontBase::AddPrivateFont(const wxString& WXUNUSED(filename))
-{
-    return false;
-}
-
-#endif // !wxHAS_PRIVATE_FONTS