How to compile in Arch Linux?

[Konstantinusz]

When I try to compile I get error messages related to json.hpp

g++ -Wall -std=c++11 -g -s -O2 -Iinclude -D_FILE_OFFSET_BITS=64 -c src/concat.cpp -o obj/concat.o
In file included from src/concat.cpp:14:
include/json.hpp:30: error: unterminated #ifndef
   30 | #ifndef INCLUDE_NLOHMANN_JSON_HPP_
      | 
src/concat.cpp:466:1: error: expected ‘}’ at end of input
  466 | }
      | ^
In file included from src/concat.cpp:14:
include/json.hpp:14944:1: note: to match this ‘{’
14944 | {
      | ^
include/json.hpp: In constructor ‘nlohmann::detail::serializer<BasicJsonType>::serializer(nlohmann::detail::output_adapter_t<char>, char, nlohmann::detail::error_handler_t)’:
include/json.hpp:14961:11: error: class ‘nlohmann::detail::serializer<BasicJsonType>’ does not have any field named ‘o’
14961 |         : o(std::move(s))
      |           ^
include/json.hpp:14962:11: error: class ‘nlohmann::detail::serializer<BasicJsonType>’ does not have any field named ‘loc’
14962 |         , loc(std::localeconv())
      |           ^~~
include/json.hpp:14963:11: error: class ‘nlohmann::detail::serializer<BasicJsonType>’ does not have any field named ‘thousands_sep’
14963 |         , thousands_sep(loc->thousands_sep == nullptr ? '\0' : * (loc->thousands_sep))
      |           ^~~~~~~~~~~~~
include/json.hpp:14963:25: error: ‘loc’ was not declared in this scope; did you mean ‘log’?
14963 |         , thousands_sep(loc->thousands_sep == nullptr ? '\0' : * (loc->thousands_sep))
      |                         ^~~
      |                         log
include/json.hpp:14964:11: error: class ‘nlohmann::detail::serializer<BasicJsonType>’ does not have any field named ‘decimal_point’
14964 |         , decimal_point(loc->decimal_point == nullptr ? '\0' : * (loc->decimal_point))
      |           ^~~~~~~~~~~~~
include/json.hpp:14965:11: error: class ‘nlohmann::detail::serializer<BasicJsonType>’ does not have any field named ‘indent_char’
14965 |         , indent_char(ichar)
      |           ^~~~~~~~~~~
include/json.hpp:14966:11: error: class ‘nlohmann::detail::serializer<BasicJsonType>’ does not have any field named ‘indent_string’
14966 |         , indent_string(512, indent_char)
      |           ^~~~~~~~~~~~~
include/json.hpp:14966:30: error: ‘indent_char’ was not declared in this scope
14966 |         , indent_string(512, indent_char)
      |                              ^~~~~~~~~~~
include/json.hpp:14967:11: error: class ‘nlohmann::detail::serializer<BasicJsonType>’ does not have any field named ‘error_handler’
14967 |         , error_handler(error_handler_)
      |           ^~~~~~~~~~~~~
include/json.hpp: In member function ‘void nlohmann::detail::serializer<BasicJsonType>::dump(const BasicJsonType&, bool, bool, unsigned int, unsigned int)’:
include/json.hpp:15011:21: error: ‘o’ was not declared in this scope
15011 |                     o->write_characters("{}", 2);
      |                     ^
include/json.hpp:15017:21: error: ‘o’ was not declared in this scope
15017 |                     o->write_characters("{\n", 2);