fmt v8 currently contains a bug that causes compilation to fail if
`FMT_USE_USER_DEFINED_LITERALS` is defined to 0. This is a regression
from earlier versions. A bug has been filed against fmt:
https://github.com/fmtlib/fmt/issues/2384
MKVToolNix defined this to 0 as earlier fmt versions caused warnings
in their code dealing with user-defined literals with
`-Wpedantic`. Luckily those warnings do not occur with v8 anymore.
The fix is to test in `configure` if the fmt version we're compiling
with is v8 or older, and only to define
`FMT_USE_USER_DEFINED_LITERALS` to 0 for earlier versions.
Fixes#3151.
