Build issues related to libanl

[zhangyi1357]

I am trying to build a Python version of a robot runtime framework that depends on the paho.mqtt.c project on manylinux with glibc 2.28, aiming to create a wheel package that can be uploaded to PyPI for easy user download.

During the execution of the above task, I encountered some problems with paho.mqtt.c, more precisely with the libanl library that paho.mqtt.c depends on. This library contains private symbols, and these symbols are also included in the generated dynamic library .so file, thus causing the auditwheel repair command to fail and preventing the creation of a wheel package with a specific tag.

However, I found this commit in the PR of this project, which suggests that the project can also be built on platforms where libanl is not present. So I tried commenting out the following content of this project, recompiling, and the resulting dynamic library file indeed did not contain the related private symbols.

    find_library(LIB_ANL anl)
    if(LIB_ANL)
      set(LIBS_SYSTEM "${LIBS_SYSTEM}" anl)
    endif()

Things seem to be progressing well at this point, but I'm unsure if removing the libanl dependency will affect the MQTT functionality. I only know that libanl is probably related to DNS resolution (and I'm not even sure about that). Information about libanl online is scarce, so I hope to get some information or help here.