Set version for shared library

Fix compiler warnings

Remove redundant semicolon

Add ringbuffer sink

Ringbuffer sink keeps user-given number of most recent log messages in
memory and returns them upon request (using the ringbuffer_sink::last
method). This can be useful for e.g. remote debugging of a running app.

"#include" <spdlog/.*> instead of "spdlog/.*"

The meaning of using quotes to #include is implementation defined, so it
may or not may be what we want. At least POSIX
(https://pubs.opengroup.org/onlinepubs/9699919799/utilities/c99.html)
says: "headers whose names are enclosed in double-quotes ( "" ) shall be
searched for first in the directory of the file with the #include line",
so not what we want since "spdlog" ends up twice in the path.