libmemif: Fixed strlcpy symbol detection.

libmemif: Fixed strlcpy symbol detection.

Type: fix

libmemif can't be compiled with the modern glibc (since 2.38)
and musl as they include strlcpy for _GNU_SOURCE by default now.

The change introduced:

- proper symbol detection for both strlcpy and memfd_create;
- bumped CMake version requirements due to soon-to-be failed
compilation for very old version;
- fixed Unity compilation on the modern compilers (it has warnings,
but compiled with `-Werror`).

Change-Id: I48f9c410aa5405174dc6b65e9c9001e8b11ba276
Signed-off-by: Alexander Slesarev <[email protected]>