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_wxusedef.h

@@ -186,6 +186,7 @@ library:
 @itemdef{wxUSE_POSTSCRIPT, Use wxPostScriptPrinter class.}
 @itemdef{wxUSE_PRINTF_POS_PARAMS, Use wxVsnprintf which supports positional parameters.}
 @itemdef{wxUSE_PRINTING_ARCHITECTURE, Enable printer classes.}
+@itemdef{wxUSE_PRIVATE_FONTS, Implement wxFont::AddPrivateFont() method.}
 @itemdef{wxUSE_PROGRESSDLG, Enables progress dialog classes.}
 @itemdef{wxUSE_PROPGRID, Use wxPropertyGrid library.}
 @itemdef{wxUSE_PROTOCOL, Use wxProtocol and derived classes.}