executing `make install` and WITH_C_API is set.

* Refine register methods, make Op can get rid of whole-archieve
* `USE_OP` before a op is used.
* Add unittest for add_op.

So that they can be found when paddle is built as a add_subdirectory project in another project.

Also initialize glog after gflags in Util.cpp initMain()

Change-Id: I09ff062b462aa76d9f7b5f97e883b92939e6dcb2