* Fix bug in MergeModel.cpp.

* Add a c-api inferface to initilize the thread environment of Paddle and add a GPU example.

* Add some note for paddle_init_thread and move the inplementation of paddle_error_string into a .cpp file.

* Add some comments.

Add version and commit information in capi config.h and use unofficial glog for Android API < 21. (#6113)

* Automatically configure the version and commit information in capi.

* Use the unofficial glog repository for building for Android (API < 21).

model inference

The engine library need to link paddle_pserver and paddle_network on linux.

By shell command

```bash
sed -i 's#ifdef PADDLE_ONLY_CPU#ifndef PADDLE_WITH_GPU#g' `find ./paddle/ -name '*.h' -o -name '*.cc' -o -name '*.cpp' -o -name '*.c' -o -name '*.cu'`
sed -i 's#ifndef PADDLE_ONLY_CPU#ifdef PADDLE_WITH_GPU#g' `find ./paddle/ -name '*.h' -o -name '*.cc' -o -name '*.cpp' -o -name '*.c' -o -name '*.cu'`
```

Remove the pserver, trainer, evaluators and some useless gradientmachines when compile mobile inference library.

Recover the mistakenly deleted WARPCTC variable in cmake.

hidden capi symbols

related codes in Paddle core.

to create a gradient machine for inference using merged model with parameters
which is genearted by `paddle merge_model`.

Fix #2567