doc: hide "__extern" from the final documentation.

doc/doxygen.cfg.in

@@ -1260,7 +1260,7 @@ INCLUDE_FILE_PATTERNS  =
 # undefined via #undef or recursively expanded use the := operator 
 # instead of the = operator.
 
-PREDEFINED             = _DOXYGEN_SKIP_ME
+PREDEFINED             = _DOXYGEN_SKIP_ME __extern=
 
 # If the MACRO_EXPANSION and EXPAND_ONLY_PREDEF tags are set to YES then 
 # this tag can be used to specify a list of macro names that should be expanded.