[clang-build] specify thread flag as part CPPFLAGS, not LDFLAGS.

newer version of clang treats the latter as a fatal error.

configure.ac

@@ -743,7 +743,8 @@ else
 	done
 fi
 
-LOG4CPLUS_LIBS="$LOG4CPLUS_LIBS -llog4cplus $MULTITHREADING_FLAG"
+LOG4CPLUS_INCLUDES="$LOG4CPLUS_INCLUDES $MULTITHREADING_FLAG"
+LOG4CPLUS_LIBS="$LOG4CPLUS_LIBS -llog4cplus"
 
 AC_SUBST(LOG4CPLUS_LIBS)
 AC_SUBST(LOG4CPLUS_INCLUDES)