Important changes:
- Switch mergerfs and vendored libfuse Makefiles to -std=c++20.
- Select the latest available GCC/G++ or toolset in container builds.
- Verify compiler C++20 support during package installation.
- Fix dynamic SysLog formatting for fmt C++20 checks.
libfuse previously initialized its debug log filepath to
"/dev/stderr", forcing the config layer to reopen stderr on every
load whether or not the user asked for it. Default the libfuse log
path to "" and have the mergerfs config layer only forward the
request to libfuse when the user supplied an explicit path.
Antonio SJ Musumeci