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.

docs/doxygen/mainpages/const_cpp.h

@@ -188,7 +188,6 @@ Currently the following symbols exist:
 @itemdef{wxHAS_IMAGES_IN_RESOURCES, Defined if <a href="http://en.wikipedia.org/wiki/Resource_(Windows)">
     Windows resource files</a> or OS/2 resource files are available on the current platform.}
 @itemdef{wxHAS_POWER_EVENTS, Defined if wxPowerEvent are ever generated on the current platform.}
-@itemdef{wxHAS_PRIVATE_FONTS, Defined if wxFont::AddPrivateFont() is implemented.}
 @itemdef{wxHAS_RADIO_MENU_ITEMS,
         Defined if the current port supports radio menu items (see wxMenu::AppendRadioItem).}
 @itemdef{wxHAS_RAW_BITMAP, Defined if direct access to bitmap data using the classes in @c wx/rawbmp.h is supported.}