[AndreT]

This is weird, but is not influenced by the code in any way.

If you were to compile without linking against mysqlclient_r (-lmysqlclient_r means the linker looks for libmysqlclient_r.so in the directories specified with -L), it would most likely compile since the MySQL headers exist, but when running the plugin, the headers wouldn't be any use without the library to actually provide these functions.

Try running ldconfig to see if refreshing the cache would solve your issue (afaik this is for runtime though, not sure, but give it a try at least).

There might be something vital I'm missing here, but if it wouldn't bother you, you could also compile a static version of the plugin (a few MBs bigger, I assume), the .a file should be in the repo as well.

Best of luck!