net-mgmt/fastnetmon: Update to 1.2.6

• Switch from PORTVERSION to DISTVERSION
• Remove USE_CXXSTD (no longer needed)
• Drop devel/json-c in LIB_DEPENDS (not used)
• Use WRKSRC_SUBDIR
• Drop -DBOOST_STACKTRACE_GNU_SOURCE_NOT_REQUIRED switch
• Remove LLVM version requirements (no longer needed)
• Use CMAKE helpers
• Rearrange Makefile for better readability
• Disable unsupported -W switches making the build a lot less noisy
• Remove bogus include paths

Based on patch submitted by Marek Zarychta

Changelog:
https://github.com/pavel-odintsov/fastnetmon/releases/tag/v1.2.6

PR: 276516
Reported by: Marek Zarychta <zarychtam@plan-b.pwste.edu.pl>
Reviewed by: farrokhi (maintainer, previous version)
Tested by: Marek Zarychta <zarychtam@plan-b.pwste.edu.pl>
Approved by: portmgr (maintainer timeout, 2+ weeks)