Cannot link tests when building as static library

[FSMaxB]

I noticed this while trying to implement #207.

This is most likely because common.h includes cJSON.c, but not entirely sure. But this needs to be fixed anyway.

The reason I did this is because I need a way to test cJSONs static functions in the unit tests, but they are not visible from outside of the library.

A fix would be to use a macro like cJSON_Private that can be changed to build a version of cJSON that exposes it's internals, so they can be tested.

I think it is ok to build a shared cJSON library with exposed internals like that, even if cJSON is built as a static library, because this is just for testing and won't be installed on the system.

[FSMaxB]

This is harder than I thought it would be.

[FSMaxB]

This is fixed on the develop branch. Although I didn't succeed in making a CJSON_PRIVATE macro.