add max files for rotating days

latest spdlog now available on MacPorts

I am the maintainer of spdlog on MacPorts, now the latest version(1.4.2) of spdlog is available on MacPorts.

docs: spdlog now available in conan center, bincrafters repo is deprecated

meson: add fallback to fmt dependency

Now `fmt` library can be used as subproject which helps with cross
compilation.

Fix invalid meson option

Fix a small problem in the basic example

The line
```
SPDLOG_TRACE("Some trace message with param {}", {});
```
which normally is discarded at the compile time since by deafult
`SPDLOG_ACTIVE_LEVEL` is at `SPDLOG_LEVEL_INFO`. If however, one goes
to `tweak.me` and sets
```
 #define SPDLOG_ACTIVE_LEVEL SPDLOG_LEVEL_TRACE
```
suddenly the basic example does not compile any more.
This commit fixes the potential problem.