Using ExternalProject_Add to build third party dependencies

[gangliao]

#932

Add a directory external under cmake.

1. Some dependencies should be optional, we first search them from system default, if not found, pull from git/svn/https, add them as external dependencies.

For instance,

swig, python, numpy, git

2. Other dependencies should be added and installed default using ExternalProject_Add .

For example,

glog, gflags, gtest, protobuf, openblas, zlib, warpctc

Pull and build remote third-party's source code is quite slow, we could add a CMake parameter USE_SYSTEM to make our CI system run faster.

[gangliao]

Currently, all external dependencies works very well under my test project https://github.com/gangliao/CodeCoverageCMake/tree/master/cmake/external

Starting to port into Paddle.