Created by: luotao1

编译单测的时候，会出现大量Flags.h和Matrix.h的warning，类似如下：

/home/luotao/Paddle/paddle/utils/Flags.h(28): warning: using-declaration ignored -- it refers to the current namespace

/home/luotao/Paddle/paddle/utils/Flags.h(29): warning: using-declaration ignored -- it refers to the current namespace

/home/luotao/Paddle/paddle/utils/Flags.h(32): warning: using-declaration ignored -- it refers to the current namespace

/home/luotao/Paddle/paddle/utils/Flags.h(33): warning: using-declaration ignored -- it refers to the current namespace

/home/luotao/Paddle/paddle/utils/Flags.h(41): warning: using-declaration ignored -- it refers to the current namespace

/home/luotao/Paddle/paddle/utils/Flags.h(43): warning: using-declaration ignored -- it refers to the current namespace

/home/luotao/Paddle/paddle/math/Matrix.h(1162): warning: overloaded virtual function "paddle::Matrix::addBias" is only partially overridden in class "paddle::GpuMatrix"

/home/luotao/Paddle/paddle/math/Matrix.h(1162): warning: overloaded virtual function "paddle::Matrix::collectBias" is only partially overridden in class "paddle::GpuMatrix"

/home/luotao/Paddle/paddle/math/Matrix.h(1538): warning: overloaded virtual function "paddle::Matrix::addBias" is only partially overridden in class "paddle::CpuMatrix"

/home/luotao/Paddle/paddle/math/Matrix.h(1538): warning: overloaded virtual function "paddle::Matrix::collectBias" is only partially overridden in class "paddle::CpuMatrix"

其中，Flags.h的warning都是在P_DECLARE_string(XXX)时出现的。