Change libfaketime path search procedure

Path is searched in following localizations:
- `LIBFAKETIME_PATH` environment variable
- default installation path when building from sources (/usr/local/lib/faketime/)
- default Ubuntu package installation path (/usr/lib/x86_64-linux-gnu/faketime/)

Warning is emitted when libfaketime from Ubuntu package repository is used,
because of missing features in older version provided by Ubuntu20's package
repository, which might be required by some tests based on TestTools.

Added installation manual, which is showed in case of configuration problem.