devel/libfmt: Update to 6.0.0
While here
- Normalize CMAKE_ARGS, USES=compiler:c++11-lib
- Pet portfmt
- Bump revision of consumers due to ABI changes
Changes: https://github.com/fmtlib/fmt/releases/tag/6.0.0
ABI: https://abi-laboratory.pro/tracker/timeline/fmt/
PR: 240141