Import nlohmann-json: a C++ JSON library

Tweaks from sthen@, caspar@ and op@
OK caspar, op and landry

textproc/nlohmann-json/Makefile

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+COMMENT =	JSON for modern C++
+
+# the "release" contain only json.hpp, and we want tests and CMake config file
+V =		3.9.1
+GH_ACCOUNT =	nlohmann
+GH_PROJECT =	json
+GH_TAGNAME =	v${V}
+DISTNAME =	nlohmann-json-${V}
+
+CATEGORIES =	devel textproc
+
+HOMEPAGE =	https://nlohmann.github.io/json/
+
+# MIT
+PERMIT_PACKAGE =	Yes
+
+# C++11
+COMPILER =		base-clang ports-gcc
+
+MODULES =		devel/cmake
+
+NO_TEST =		Yes
+
+.include <bsd.port.mk>

textproc/nlohmann-json/distinfo

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+SHA256 (nlohmann-json-3.9.1.tar.gz) = TPDfaXMUlGaL3WRg7YyyabaN6cGa2MJ6vCTNcmBbLVs=
+SIZE (nlohmann-json-3.9.1.tar.gz) = 6882818

textproc/nlohmann-json/pkg/DESCR

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+There are myriads of JSON libraries out there, and each may even have its reason
+to exist. This class had these design goals:
+
+  * Intuitive syntax. In languages such as Python, JSON feels like a first class
+    data type. Here all the operator magic of modern C++ was used to achieve
+    the same feeling in code.
+
+  * Trivial integration. The whole code consists of a single header file
+    json.hpp. That's it. No library, no subproject, no dependencies, no complex
+    build system. The class is written in vanilla C++11. All in all, everything
+    should require no adjustment of your compiler flags or project settings.
+
+  * Serious testing. The class is heavily unit-tested and covers 100% of the
+    code, including all exceptional behavior. Furthermore, it's checked with
+    Valgrind and the Clang Sanitizers that there are no memory leaks. Google
+    OSS-Fuzz additionally runs fuzz tests against all parsers 24/7, effectively
+    executing billions of tests so far. To maintain high quality, the project is
+    following the Core Infrastructure Initiative (CII) best practices.

textproc/nlohmann-json/pkg/PLIST

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+include/nlohmann/
+include/nlohmann/json.hpp
+lib/cmake/
+lib/cmake/nlohmann_json/
+lib/cmake/nlohmann_json/nlohmann_jsonConfig.cmake
+lib/cmake/nlohmann_json/nlohmann_jsonConfigVersion.cmake
+lib/cmake/nlohmann_json/nlohmann_jsonTargets.cmake
+lib/pkgconfig/nlohmann_json.pc