> % mariadb_config --cflags
> -I/usr/include/mariadb -I/usr/include/mariadb/mysql

I get similar, both .../include and .../include/mysql are in my include path,
so both #include "mysql/mysql.h" and #include "mysql.h" work.

I added a message to cmake to report the MySQL CFLAGS and libraries, so next time
this is a problem, we can see what happened from the cmake output:

4ed0:midas olchansk$ make cmake | grep MySQL
...
-- MIDAS: Found MySQL version 10.4.16
-- MIDAS: MySQL CFLAGS: -I/opt/local/include/mariadb-10.4/mysql;-I/opt/local/include/mariadb-
10.4/mysql/mysql and libs: -L/opt/local/lib/mariadb-10.4/mysql/ -lmariadb

K.O.