编译单测时的warning

[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。

[wangkuiyi]

@luotao1 @reyoung We are going to fix #721. In this work, we will have to remove all global variables, particularly, global flags definitions. In this work, we will remove our own flag definition logic, and use gflags literally instead.