www/libnghttp2: Update to 1.51.0

* Switch to CMake
* Modernize Makefile to follow Porters Handbook more closely

Poudriere test run by bofh (thanks!)

Changelog: https://github.com/nghttp2/nghttp2/releases/tag/v1.51.0

PR:		268536
Approved by:	portmgr (maintainer timeout, 2+ weeks)