Add GST_USING_PRINTF_EXTENSION to gstconfig.h so that we know whether we can use G_GNUC_PRINTF in other header files ...

Original commit message from CVS:
* configure.ac:
* docs/gst/gstreamer-sections.txt:
* gst/gstconfig.h.in:
* gst/gstelement.h:
* gst/gstinfo.h:
Add GST_USING_PRINTF_EXTENSION to gstconfig.h so that we know
whether we can use G_GNUC_PRINTF in other header files and at
least check the printf format/arguments of debug messages and
GST_ELEMENT_ERROR messages when the printf extension is not
being used.
Replace more tabs with spaces in gstinfo.h and remove two spurious
function declarations in GST_DISABLE_DEBUG part with macros.