cmake: fix symbol visibility and install pkg-config file

[Lekensteyn]

Patches to improve make the CMake build be similar to CMake:

• cmake: fix symbol visibility Avoid exposing unexported symbols, matches autotools behavior.
• CMake: install pkg-config files for parity with autotools

Fixes some of the issues from https://gitlab.linphone.org/BC/public/bcg729/issues/2

[jeannotlapin]

merged on the main repository

Note: symbol visibility maintained when building tests to be able to test inner functions