While trying to troubleshoot this issue, I ran into a few other problems as well. The main problem is that it failed to link to the bundled jsoncpp for the server component. I edited the Makefile so it would link to the system jsoncpp, and it compiled successfully but did not run. However, in order to finish compiling, I also had to disable NLS because the installation failed due to the missing translation files. And that was just trying to compile the server. Enabling the client introduced a different compilation failure.

Anyway, one thing at a time. I'm running 12.1-STABLE r363327. The main issue I ran into while trying to compile the server is as follows:

ld: error: undefined symbol: Json::Value::operator=(Json::Value&&)
>>> referenced by mods.cpp
>>>               src/CMakeFiles/minetestserver.dir/content/mods.cpp.o:(ModMetadata::save(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&))
>>> referenced by mods.cpp
>>>               src/CMakeFiles/minetestserver.dir/content/mods.cpp.o:(ModMetadata::load(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&))
>>> referenced by c_content.cpp
>>>               src/CMakeFiles/minetestserver.dir/script/common/c_content.cpp.o:(read_json_value(lua_State*, Json::Value&, int, unsigned char))
>>> referenced by c_content.cpp
>>>               src/CMakeFiles/minetestserver.dir/script/common/c_content.cpp.o:(read_json_value(lua_State*, Json::Value&, int, unsigned char))
>>> referenced by c_content.cpp
>>>               src/CMakeFiles/minetestserver.dir/script/common/c_content.cpp.o:(read_json_value(lua_State*, Json::Value&, int, unsigned char))
>>> referenced by c_content.cpp
>>>               src/CMakeFiles/minetestserver.dir/script/common/c_content.cpp.o:(read_json_value(lua_State*, Json::Value&, int, unsigned char))
>>> referenced by l_util.cpp
>>>               src/CMakeFiles/minetestserver.dir/script/lua_api/l_util.cpp.o:(ModApiUtil::l_parse_json(lua_State*))
>>> referenced by convert_json.cpp
>>>               src/CMakeFiles/minetestserver.dir/convert_json.cpp.o:(fetchJsonValue(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, std::__1::vector<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, std::__1::allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > > >*))
>>> referenced by convert_json.cpp
>>>               src/CMakeFiles/minetestserver.dir/convert_json.cpp.o:(fastWriteJson(Json::Value const&))
>>> referenced by remoteplayer.cpp
>>>               src/CMakeFiles/minetestserver.dir/remoteplayer.cpp.o:(RemotePlayer::serializeExtraAttributes(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >&))
>>> referenced 41 more times
>>> did you mean: Json::Value::operator=(Json::Value)
>>> defined in: lib/jsoncpp/libjsoncpp.a(jsoncpp.cpp.o)

ld: error: undefined symbol: Json::Value::operator=(Json::Value const&)
>>> referenced by c_content.cpp
>>>               src/CMakeFiles/minetestserver.dir/script/common/c_content.cpp.o:(read_json_value(lua_State*, Json::Value&, int, unsigned char))
>>> referenced by c_content.cpp
>>>               src/CMakeFiles/minetestserver.dir/script/common/c_content.cpp.o:(read_json_value(lua_State*, Json::Value&, int, unsigned char))
>>> referenced by serverlist.cpp
>>>               src/CMakeFiles/minetestserver.dir/serverlist.cpp.o:(ServerList::getOnline())
>>> referenced by serverlist.cpp
>>>               src/CMakeFiles/minetestserver.dir/serverlist.cpp.o:(ServerList::serializeJson(std::__1::vector<Json::Value, std::__1::allocator<Json::Value> > const&))
>>> referenced by serverlist.cpp
>>>               src/CMakeFiles/minetestserver.dir/serverlist.cpp.o:(std::__1::vector<Json::Value, std::__1::allocator<Json::Value> >::insert(std::__1::__wrap_iter<Json::Value const*>, Json::Value const&))
>>> referenced by tool.cpp
>>>               src/CMakeFiles/minetestserver.dir/tool.cpp.o:(ToolGroupCap::toJson(Json::Value&) const)
>>> referenced by tool.cpp
>>>               src/CMakeFiles/minetestserver.dir/tool.cpp.o:(ToolCapabilities::serializeJson(std::__1::basic_ostream<char, std::__1::char_traits<char> >&) const)
>>> referenced by tool.cpp
>>>               src/CMakeFiles/minetestserver.dir/tool.cpp.o:(ToolCapabilities::serializeJson(std::__1::basic_ostream<char, std::__1::char_traits<char> >&) const)

ld: error: undefined symbol: Json::StreamWriterBuilder::operator[](std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&)
>>> referenced by convert_json.cpp
>>>               src/CMakeFiles/minetestserver.dir/convert_json.cpp.o:(fastWriteJson(Json::Value const&))

ld: error: undefined symbol: Json::ValueIteratorBase::deref()
>>> referenced by serverlist.cpp
>>>               src/CMakeFiles/minetestserver.dir/serverlist.cpp.o:(ServerList::getOnline())
>>> referenced by tool.cpp
>>>               src/CMakeFiles/minetestserver.dir/tool.cpp.o:(ToolCapabilities::deserializeJson(std::__1::basic_istream<char, std::__1::char_traits<char> >&))
>>> referenced by tool.cpp
>>>               src/CMakeFiles/minetestserver.dir/tool.cpp.o:(ToolCapabilities::deserializeJson(std::__1::basic_istream<char, std::__1::char_traits<char> >&))
c++: error: linker command failed with exit code 1 (use -v to see invocation)
ninja: build stopped: subcommand failed.
===> Compilation failed unexpectedly.
Try to set MAKE_JOBS_UNSAFE=yes and rebuild before reporting the failure to
the maintainer.
*** Error code 1

Stop.
make[1]: stopped in /usr/ports/games/minetest
*** Error code 1

Stop.
make: stopped in /usr/ports/games/minetest