gst/gst.c: Don't use GST_INFO before the debug system is actually initialised (shouldn't do any harm, but won't print...

Original commit message from CVS:
* gst/gst.c:
Don't use GST_INFO before the debug system is actually initialised
(shouldn't do any harm, but won't print anything either, so we can
just as well remove it).
* gst/gstinfo.h:
GST_CAT_LEVEL_LOG_valist(), which is our inline helper function for
compilers that don't support variadic macros (such as MSVC), should
check for debug_level <= __gst_debug_min as well, since that's the
function called from all the level-specific GST_CAT_*_LOG_OBJECT()
inline helper functions. Should improve performance a bit, but also
makes sure uses of GST_INFO et.al are ignored if the debugging
system isn't initialised yet (instead of printing an assertion
failure).