build: don't try to link pthread on Windows
build: don't try to link pthread on Windows

Fluent Bit uses "pthread" to manage logging workers, both on Unix and
Windows. However, since Windows does not have a native pthread support,
we instead use the emulation layer in mk_core/external/winpthreads.c.

This means we should not try to link pthread by -lpthread on Windows,
or the linker will emit the following warnings.

    warning LNK4044: unrecognized option '/lpthread'; ignored

Signed-off-by: Fujimoto Seiji <ef879192bb1e41d519c82cf76440dc8e9a530806@clear-code.com>

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