The src/Makefile.am hardcodes -I/usr/kerberos/include.  When cross compiling it should be including -I$(SYSROOT)//usr/kerberos/include.

I patched configure.ac and Makefile.am, taking advantage of the existing  --with-sysroot configure option.

The Gentoo bug report: https://bugs.gentoo.org/654374
The patch to configure.ac and Makefile.am: https://654374.bugs.gentoo.org/attachment.cgi?id=528968

I was unable to find a bug reporting system for stunnel.  So I guess this is it.  Please source this fix.

---
Joe Harvell