Compilation of PaddlePaddle's C_API meets error

[gongweibao]

When I was compiling the C-API of PaddlePaddle, I got an error:

CMake Error at cmake/generic.cmake:185 (add_dependencies):
  The dependency target "python" of target "paddle_pybind" does not exist.
Call Stack (most recent call first):
  paddle/framework/CMakeLists.txt:40 (cc_library)

And the CI environment in our company meets the same error.

[gongweibao]

When we build the PaddlePaddle inference library, a.k.a. the C-API, we'd like to disable Python, because we don't need to generate the training system's Python API.

However, in the newly added paddle/framework package, we have a rule building the Python binding which links to libpython. This rule would fail when we are building the inference library and set WITH_PYTHON and WITH_SWIG_PY as OFF.

The solution is

1. Enclose the Python binding rule by checking if Python is enabled,
2. Control Python enabling parameters in build.sh.

[gongweibao]

感谢益哥手把手的改动!