Although an option to use the system's MIT Kerberos implementation is available, there is no obvious way to request use of the already installed Heimdal -- Samba's bundled code is used unconditionally.
Please, provide the necessary configure-support -- and default to system Heimdal, unless explicitly asked to use a different Kerberos implementation.
There is a configure option --with-system-heimdalkrb5
Unfortunately that broke with Samba 4.10. Bugzilla 14179 is
tracking the fix for this breakage.