Build system: No '--disable-shared'

The agent and unit tests can now be compiled without requiring
`--disable-shared' at configuration time.

The agent requires the logger functions to have default visibility instead of
hidden.  Thus, `jami::logger::[v]log' can be considered part of the public API.

The unit tests however require hidden symbols.  Thus, we link the tests against
a static version of libjami instead.

Change-Id: I59d9e67679766e0310a19f9a879c06a31c5124c4