su_global_log.c: disable warning on SU_DEBUG[] (Doxygen-only variable)

Patch from Michael Jerris.

darcs-hash:20080121171425-65a35-ed1da84e0e438f4e447963947d89e253a44bd596.gz

libsofia-sip-ua/su/su_global_log.c

@@ -38,6 +38,7 @@
 #include <sofia-sip/su_log.h>
 #include <sofia-sip/su_debug.h>
 
+#ifdef DOXYGEN_ONLY
 /**@var SU_DEBUG
  *
  * Environment variable determining the debug log level for @b su module.
@@ -48,6 +49,7 @@
  * @sa <sofia-sip/su_debug.h>, su_log_global
  */
 extern char const SU_DEBUG[];
+#endif
 
 #ifdef SU_DEBUG
 #define SU_DEBUG_ SU_DEBUG