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