Use cmake file instead of find_library to link against trace plugin

[lsk567]

The overall goal of this PR is to support a new trace plugin that streams data to the Xronos dashboard.

The existing mechanism to link against a trace plugin library is through find_library, which requires the plugin library to be located at the root of the generated C project (i.e., LF_ROOT). This requires using the files target property to copy the trace plugin library to LF_ROOT.

While this approach works for a small, self-contained library, this approach would not work if a library has its own dependencies, as the library needs to link against them, which find_library does not do.

A more robust approach is to pull in the plugin library dependencies using a custom cmake file via the cmake-include target property.

This PR:

1. removes find_library and adds a message for the user to provide the cmake file;
2. makes the default plugin compilable using build.sh.

[edwardalee]

This looks good to me, although without an example, it is hard to see how a user could figure out how to use the trace plugin mechanism.

[edwardalee]

Could this possibly point to an example? Perhaps we could add a test in the lingua-franca repo that uses the default trace implementation as if it were an externally provided trace implementation?

[lhstrh]

Is it really true that the files target property must be used? I haven't seen that in examples. When you build a shared library and install it (i.e., put it in a location where find_library will find it, such as /usr/local/lib), no files need to be copied.

If transitive dependencies are the concern, would find_package do the trick?