Makefile: Always use SED variable

Add Xcode 12 CI

We need to have a proper sed, even on MacOSX. So let's use the variable
introduced in 191aa0fd (🔧 overworked maintaner targets,
2019-03-28) in more places.

Add ordered_json specialization with ordered object keys

Make library work with Clang on Windows

 Conflicts:
	include/nlohmann/detail/input/binary_reader.hpp
	include/nlohmann/detail/input/json_sax.hpp
	include/nlohmann/detail/input/lexer.hpp
	include/nlohmann/detail/input/parser.hpp
	include/nlohmann/detail/json_pointer.hpp
	include/nlohmann/detail/output/serializer.hpp
	include/nlohmann/json.hpp
	single_include/nlohmann/json.hpp

 Conflicts:
	single_include/nlohmann/json.hpp

Simple ordered_json that works on all supported compilers

Co-authored-by: Niels Lohmann <niels.lohmann@gmail.com>

Add pkg-config file

Make assert configurable via JSON_ASSERT

The meson builds install a pkg-config file, but the cmake builds don't.
This adds a pkg-config file to the cmake builds that is functionally
equivalent to the one generated from meson.

Fix regression from #2181