We are working to move the default version of GCC in ports (lang/gcc) from GCC 4.8 to GCC 4.9. Sadly something goes wrong with xen-tools when using this newer version of GCC: http://package22.nyi.freebsd.org/data/101amd64-default-PR196712/2015-10-25_21h38m31s/logs/errors/xen-tools-4.5.1_1.log One approach might be USE_GCC=4.8 for the time being if this cannot be addressed easily.
*** This bug has been marked as a duplicate of bug 204150 ***