Properly handling SPDLOG_PREVENT_CHILD_FD

Removed check for posix version so that missing O_CLOEXEC leads to
compiler error.

Removed extra function since it hat no real purpose anymore.

Error behavior between Windows and Unix now equivalent.

Using the SPDLOG_PREVENT_CHILD_FD option there where still a race when
a other thread was using fork and exec in between the call to fopen and fcntl.

Using open and O_CLOEXEC when possible prevents this race.

I have no idea if this problem  exists on Windows.

Enable running the tests against an installed copy of spdlog

CMakeLists.txt: put GNUInstallDirs after set project language

Don't run spdlog-utests and spdlog-utests-ho in parallel

spdlog-utests and spdlog-utests-ho use the same log output directory, so some tests will randomly fail when run in parallel.

tests/utils.h: removed duplicate entry for count_lines()

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.