Versions prior to this didn't compile on OpenBSD due to unconditional
use of the non-standard strtod_l() function.
The fmt::MemoryWriter API has been removed in the intervening
versions, so replace its use with fmt::memory_buffer and fmt::format_to.
The library also no longer provides the fmt::fmt ALIAS, so define
it in externals/CMakeLists.txt.
fd79b70a87