CMake: Try to use pkg-config for hidapi on macOS

[muellermartin]

Try to use CMake's module for pkg-config to find an installed version of HIDAPI and fall back to the current solution (local copy of HIDAPI in the source tree).

This fixes #140.

[szszszsz]

Hi!
As mentioned in the source issue, was the libnitrokey build linked against homebrew's hidapi working for you? I had different experiences in the past, and recently similar issue has shown in other project: d-e-s-o/nitrocli#39.

[muellermartin]

As mentioned in the related issue, the build currently works for me (compiles and test_safe passes). Maybe I will look into the issue to find out why it did not work in the past.

[szszszsz]

I see. Which version of the homebrew package of libhidapi do you use?

[muellermartin]

Which version of the homebrew package of libhidapi do you use?

As stated in the related issue, I'm using stable 0.8.0-rc1 (currently the latest version).

[szszszsz]

LGTM
Will be merged after brief tests / rebase