During BSDCan 2018 the intention to update OpenSSL in base to 1.1.x branch was documented. Intention is to update 12-STABLE to current 1.1.0 and subsequently update it to 1.1.1 when that is released. Build log in https://keg.brnrd.eu/data/111amd64-default-openssl110/2018-06-11_10h42m37s/logs/errors/xmlsec1-1.2.25.log Not sure if this is an error with OpenSSL 1.1 or with the port itself.
Issue is bigger than just openssl-devel. Also fails with regular OpenSSL 1.0.2o from ports, see https://keg.brnrd.eu/data/111amd64-default-openssl/2018-06-12_14h10m42s/logs/errors/xmlsec1-1.2.25.log
Version 1.2.27 exist. https://www.aleksey.com/xmlsec/news.html October 23 2018 The XML Security Library 1.2.27 release includes the following changes: Added AES-GCM support for OpenSSL and MSCNG (snargit). Added DSA-SHA256 and ECDSA-SHA384 support for NSS (vmiklos). Added RSA-OAEP support for MSCNG (vmiklos). Continuous build integration in Travis and Appveyor. Several other small fixes (more details).
Current port compiles fine on 12.1 stable.