json.hpp wants it, but it isn't always available. Rather than patch
json.hpp directly to remove the dependency, provide a json.h wrapper
header that subs in std::experimental::string_view using preprocessor
magic. All the consumers of json.hpp are in src/web_service, so the
wrapper header is placed there as well.
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.
swap{16,32,64} are defined as macros on the two, but client code
tries to invoke them as Common::swap{16,32,64}, which naturally
doesn't work. This hack redefines the macros as inline functions
in the Common namespace: the bodies of the functions are the
same as the original macros, but relying on OS-specific
implementation details like this is of course brittle.
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