[2340] Revert "[2340] specify thread flag as part CPPFLAGS, not LDFLAGS."

This reverts commit daf81c88.

I believe the sense of the change is correct, but it has effects on
other OSes, so it's probably better to use clang specific workaround
(will commit it later).

configure.ac

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