Export (library) definitions

[vanrein]

We currently do not export definitions using either of:

• CMake
• PkgConfig

We should probably do both.

A noteworthy concern is the distinction between libtlspool.* and libtlspool_async.* -- perhaps we need to make two exports. There may be another variant later, libtlspool_remote.*, which would be a drop-in replacement for libtlspool_async.*.

The current synchronous library is not use the asynchronous library, but that may change in the future, if we re-implement the former in terms of the latter.