Sorry for being unclear on this point!
No they are part of centos, but I compiled from source trying to reproduce your error and it seems like the flag:
is the flag to add to get rid of error below:
You have to check for similar package for slackware, and especially the develeopment packages.
checking krb5.h presence... yes
checking for krb5.h... yes
checking com_err.h usability... no
checking com_err.h presence... no
checking for com_err.h... no
configure: error: header file <com_err.h> is required for Kerberos 5
If the header is somewhere else try:
find / -name com.err.h and change --with-includes=/path_to_your_headers
and this packages should also include getting rid of your error:
configure: error: could not find function 'krb5_sendauth' required for Kerberos 5
Hope this helps