textproc/libxml++: Update to 5.2.0

• Add USES= compiler:c++17-lang
• Remove USES= python:build
• Add LICENSE_FILE
• Fix install path of .pc file
• Make some minor changes to port Makefile

Changelog:
https://github.com/libxmlplusplus/libxmlplusplus/releases/tag/5.2.0

PR: 279757
Approved by: portmgr (maintainer timeout, 2+ weeks)