Merge branch 'fixeria/lib' into master

The previous GAPK implementation was represented by a single
executable. So, all audio transcoding operations were available
only via calling the 'gapk' binary. This approach didn't allow
external applications to benefit from using GAPK API directly.

The following set of changes separates the common code into a
shared library called 'libosmogapk', linking the 'gapk' binary
against it:

  - 95e6664 Introduce a shared 'libosmogapk' library
  - 30209ce Install GAPK headers to '${includedir}/osmocom/gapk/'
  - a8d4657 Add an 'osmo_gapk' prefix to the exposed symbols
  - 40d59f1 Add a pkg-config manifest for libosmogapk
  - 4f0a47d Add the symbol export map for libosmogapk

All memory management operations are now based on talloc library:

  - 3c20dac libosmogapk: use talloc for memory management
  - 5cabe1e osmo-gapk: use talloc for memory management

Integrated Osmocom logging framework:

  - c35ba8a libosmogapk: use Osmocom logging framework
  - 4b7cd2c osmo-gapk: drop useless printf calls
  - 0fe18af osmo-gapk: use Osmocom logging framework
  - 11943bf osmo-gapk: adjust application verbosity

Integrated GNU Autotest environment and basic test coverage:

  - f069eb3 Init automake test environment
  - 1fe6a9b tests: add procqueue test
  - 3e9e57f tests: add pq_file test
  - 9d2b15d tests: add pq_rtp test
  - f59f3f1 tests: add format / codec transcoding tests

For more details, see commits history.

Change-Id: I3c6d4a9d326ee49153e4ad83823d094831c112da