Conditional test-case compilation based on installed libraries

[vaind]

WRT code compilation: the compiler needs to have objectbox & flatbuffers/flatc headers on include path. Temporarily, I've installed them on my PC to system folders (/usr/local/lib) and the same can be done for the CI, until we decide if/how we're going to integrate those headers with the generator so the generator could output them in along with objectbox headers

The tests would:

• use flatbuffers headers already present in the repo
• check whether flatcflatcc is installed (by doing a dummy compilation) to enable/disable per-testcase compilation of generated C code
• [optional] check whether objectbox is installed, similar to the previous point but for C++ generated code
• ensure CI does have both flatcc and flatbuffers installed and the compilation steps are executed

Originally posted by @vaind in https://github.com/objectbox/objectbox-generator/pull/1#issuecomment-645994029

[greenrobot]

check whether flatc is installed

We'll use our embedded flatc, right? You probably refer to flatcc?

check whether objectbox is installed

To clarify: tests should download objectbox.h and lib if not yet present.