tcp_sink implementation for fluentbit

Make tests support empty SPDLOG_EOL

adding additional log overload to prevent unnecessary fmt::format

Add eventlog_sink for logging to Windows Event Log (local only)

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.

calling log with a string_view as msg called
```
 template<typename... Args>
    void log(source_loc loc, level::level_enum lvl, string_view_t fmt, const Args &... args)
```

instead of
```
template<class T, typename std::enable_if<std::is_convertible<const T &, spdlog::string_view_t>::value, T>::type * = nullptr>
    void log(source_loc loc, level::level_enum lvl, const T &msg)
```

which lead to an unnecessary call to fmt::format

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()