tunnelbroker: Use pkg-config to find dependencies

Summary:
Installs the crypto++ and uv libraries to system install paths and use
pkg-config to find them. This is better than hard-coding the paths in
CMakeLists.

Test Plan: Run tunnelbroker test and launch scripts

Reviewers: ashoat, geekbrother, karol-bisztyga, varun

Reviewed By: ashoat, geekbrother, karol-bisztyga, varun

Subscribers: palys-swm, Adrian, atul, benschac

Differential Revision: https://phabricator.ashoat.com/D3237