Fix libmodbus build for mingw#3

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jimklimov wants to merge 9 commits intonetworkupstools:rtu_usbfrom

jimklimov:fix-mingw-build

Commits on Jul 17, 2025

configure.ac, tests/unit-test-(server|client).c: on WIN32 use setvbuf() to auto-flush so logs are comprehensible
jimklimov
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tests/unit-test-server.c: fflush() during the loop to minimize broken console logs on WIN32
jimklimov
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tests/unit-test-client.c: sleep more and flush again before the "7ms > 5ms" test case
jimklimov
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tests/unit-test-client.c: flush before "4/5 modbus_write_and_read_registers" so the buffer does not contaminate the result on some systems
jimklimov
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tests/unit-test-client.c: on WIN32 replace "7ms > 5ms" test case with "33ms > 20ms"
jimklimov
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tests/unit-test-client.c: apply "33ms > 20ms" test also to FreeBSD and OpenBSD
jimklimov
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tests/unit-test-client.c: separate *BSD tests with "66ms > 20ms" into their own category
jimklimov
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tests/unit-test-client.c: revise test case "Adapted byte timeout (Xms > Yms)" to increase the timeouts and fit all laggy systems better
jimklimov
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src/modbus.c: _modbus_receive_msg(): if ctx->backend->select() failed on WIN32, and we follow MODBUS_ERROR_RECOVERY_LINK, do modbus_flush(ctx)
jimklimov
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