security/xmlsec1: Update to 1.2.38

• Use GitHub as primary site as it's much faster and fall back to author's website if GitHub is unavailable
• Default to OpenSSL and NSS backend, GnuTLS is optional and can co-exist
• Rework Makefile to more closely follow Porters Handbook

Changelog: https://github.com/lsh123/xmlsec/releases/tag/xmlsec-1_2_38

PR: 273108
Approved by: portmgr (blanket, maintainer timeout 2+ weeks)
Sponsored by: Blinkinblox