Before attempting to port MySQL to other operating systems, check the list of currently supported operating systems first. See https://www.mysql.com/support/supportedplatforms/database.html.
If you create a new port of MySQL, you are free to copy and distribute it under the GPL license, but it does not make you a copyright holder of MySQL.
A working POSIX thread library is needed for the server.
To build MySQL from source, your system must satisfy the tool requirements listed at Installing MySQL from Source.
If you run into problems with a new port, you may have to do some debugging of MySQL! See Debugging a MySQL Server.
Before you start debugging mysqld, first get
the test program
mysys/thr_lock to work. This
ensures that your thread installation has even a remote chance
The MySQL source code contains internal documentation written using Doxygen. This documentation is useful for understanding how MySQL works from a developer perspective. The generated Doxygen content is available at https://dev.mysql.com/doc/index-other.html. It is also possible to generate this content locally from a MySQL source distribution using the instructions at Generating MySQL Doxygen Documentation Content.