Fix build on gcc <= 4.9, #512.
Fix build on gcc <= 4.9, #512.

Workaround a bug in gcc "-Wshadow is too strict / has false positives"
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57709

This should fix build on Ubuntu 14.04, Debian 7&8, CentOS 7.

westfly/muduo,westfly/muduo,westfly/muduo