Hi,
with the recent fix of the CMakeLists.txt, it seems like another bug surfaced. 
In midas/progs/mlogger.cxx:48/49, the mysql header files are included without a 
prefix. However, mysql.h and mysqld_error.h are in a subdirectory, so for our 
systems, the lines should be
  48 #include <mysql/mysql.h>
  49 #include <mysql/mysqld_error.h>
This is the case with MariaDB 10.5.5 on OpenSuse Leap 15.2, MariaDB 10.5.5 on 
Fedora Workstation 34 and MySQL 5.5.60 on Raspbian 10.

If this problem occurs for other Linux/MySQL versions as well, it should be 
fixed in mlogger.cxx and midas/src/history_schema.cxx.
If this problem only occurs on some distributions or MySQL versions, it needs 
some more differentiation than #ifdef OS_UNIX. 

Also, this somehow seems familiar, wasn't there such a problem in the past?