Make building own libsamplerate and pybind11 optional

[uis246]

No description provided.

[uis246]

Fixes #24

Currently it does not check if libsamplerate is installed. It has to be done with pkgconf, I think.

[fakufaku]

Hi @uis246 , thanks for the contribution.

While it is nice to have the option to use the local libraries, you set the default to ON, which means the install now fails on all systems that do not have it. Libsamplerate is not such a popular library, so keeping the option to OFF is a better default I think.
Could you please make the modification?

As an improvement, would it be possible to fall back to OFF (not using local libraries) if the library cannot be found? I suppose that cmake should have such a mechanism, but I am not an expert myself.

[fakufaku]

The current CI tests only check the static linking of libsamplerate.
Would it be possible for you to write a CI test for your option?

[uis246]

Would it be possible for you to write a CI test for your option?

Not sure how and not sure what exactly it should test.

[fakufaku]

Sorry I was so slow to come back to this.
You could add a section in the CI where you install the external libsamplerate library and then compile with the option set to OFF.