add pkg-config support

[pabigot]

First, thanks for creating this system. I need a C++17 RabbitMQ client in a systemd daemon using sd-event and the contortions I was going through to use rabbitmq-c were disheartening. I have high hopes for this alternative.

I also need pkg-config support, but have been unable to extract the relevant commits from pull request #170. I'm creating this issue for tracking, and will immediately be submitting a PR that provides only that feature.

The only "invasive" change is to add CMake variables for the version number, and to increment the patch version to 3.0.2. The rationale for the increment now is that anything built from current master is no longer 3.0.1 so the versioning should be different. This can be mooted in the PR or here.

[pabigot]

Wow, fast. Thanks. Apparently I haven't managed to learn to close github issues in commit comments correctly yet.