security/p5-GSSAPI fails to build if kit-krb5 installed

[David Brownlee <abs%absd.org@localhost>]

I have a package which is trying to pull in security/p5-GSSAPI as a
dependency, but its failing to build because mit-krb5 is installed.
This is on NetBSD-7_BETA amd64.

It fails trying to include gssapi/gssapi_generic.h, which is in the
mit-krb5 package, but not in the base gssapi includes.
GSSAPI.xs:34:35: fatal error: gssapi/gssapi_generic.h: No such file or directory
 #include <gssapi/gssapi_generic.h>

Interestingly "make show-var VARNAME=KRB5_TYPE" reports heimdal, so it
looks like an interaction between the os builtin heimdal and mit-krb5
package confusing the package configure...