devel/root: Update to 6.26/10
- Port now compiles against C++17
- Added build of ROOT v7 preview features as default build option
- Added dependencies on devel/nlohmann-json, graphics/ftgl, print/gl2ps and devel/xxhash so ROOT does not use builtin versions of these libraries
- ROOT now builds on FreeBSD 12.3 (uses LLVM/Clang 10.0.1) -- a bit of .modulemap hacking was necessary; tested in poudriere jail on my own system
- ROOT *should* build on arm64 -- some files are not installed on arm64 and some files are named differently
ChangeLog: https://root.cern/doc/v626/release-notes.html#release-6.2610
PR: 268701