live555: add support for building dynamic libraries

Both config.linux and config.linux-with-shared-libraries already exist
in upstream code. We are only appending to these files to override
some variables. The linux-with-shared-libraries variant defines a few
additional variables needed for dynamic linking (library version,
installation target).

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>