There are several places where connector-c is redefining or implementing things on Windows which are already available when compiling with mingw-w64 (the windows gcc port). It would be good to distinguish between different compilers on Windows.

Downstream patches are here: https://github.com/Alexpux/MINGW-packages/tree/master/mingw-w64-libmariadbclient

For example libmariadb/my_pthread.c is implementing pthreads which have been natively supported in mingw-w64 which leads to linking errors. An easy fix is to #ifdef _MSC_VER instead of #ifdef _WIN32.