gnu: Remove duplicate nlohmann-json-cpp package.

* gnu/packages/serialization.scm (nlohmann-json-cpp): Move variable ...
* gnu/packages/cpp.scm (nlohmann-json-cpp): ... here.  Mark as deprecated by
JSON-MODERN-CXX.
* gnu/packages/jupyter.scm (xeus)[inputs]: Change NLOHMANN-JSON-CPP to
JSON-MODERN-CXX.
* gnu/packages/video.scm (mkvtoolnix)[inputs]: Likewise.

gnu/packages/cpp.scm

@@ -253,6 +253,9 @@ (define-public json-modern-cxx
 intuitive syntax and trivial integration.")
     (license license:expat)))
 
+(define-public nlohmann-json-cpp
+  (deprecated-package "nlohmann-json-cpp" json-modern-cxx))
+
 (define-public xtl
   (package
     (name "xtl")

gnu/packages/jupyter.scm

@@ -30,7 +30,6 @@ (define-module (gnu packages jupyter)
   #:use-module (gnu packages pkg-config)
   #:use-module (gnu packages python)
   #:use-module (gnu packages python-xyz)
-  #:use-module (gnu packages serialization)
   #:use-module (gnu packages time)
   #:use-module (gnu packages tls))
 
@@ -168,7 +167,7 @@ (define-public xeus
        ("python-jupyter-client" ,python-jupyter-client)))
     (inputs
      `(("xtl" ,xtl)
-       ("nlohmann-json-cpp" ,nlohmann-json-cpp)
+       ("json-modern-cxx" ,json-modern-cxx)
        ("cppzmq" ,cppzmq)
        ("zeromq" ,zeromq)
        ("openssl" ,openssl)

gnu/packages/serialization.scm

@@ -40,6 +40,7 @@ (define-module (gnu packages serialization)
   #:use-module (gnu packages boost)
   #:use-module (gnu packages check)
   #:use-module (gnu packages compression)
+  #:use-module (gnu packages cpp)
   #:use-module (gnu packages databases)
   #:use-module (gnu packages documentation)
   #:use-module (gnu packages gcc)
@@ -378,40 +379,6 @@ (define-public libbson
 it is comparable to protobuf.")
     (license license:asl2.0)))
 
-(define-public nlohmann-json-cpp
-  (package
-    (name "nlohmann-json-cpp")
-    (version "3.7.0")
-    (source (origin
-              (method git-fetch)
-              (uri (git-reference
-                    (url "https://github.com/nlohmann/json.git")
-                    (commit (string-append "v" version))))
-              (file-name (git-file-name name version))
-              (sha256
-               (base32
-                "0v7xih4zjixxxfvkfbs7a8j9qcvpwlsv4vrkbyns3hc7b44nb8ap"))))
-    (build-system cmake-build-system)
-    (native-inputs
-     ;; Integer overflow tests like those from
-     ;; <https://github.com/nlohmann/json/issues/1447> fail when building with
-     ;; gcc@5.  Thus, build with a newer GCC.
-     `(("gcc" ,gcc-9)))
-    (arguments
-     '(#:phases (modify-phases %standard-phases
-                  (add-before 'build 'unset-path-variables
-                    (lambda _
-                      (unsetenv "C_INCLUDE_PATH")
-                      (unsetenv "CPLUS_INCLUDE_PATH")
-                      #t)))))
-    (home-page "https://nlohmann.github.io/json/")
-    (synopsis "JSON library for C++")
-    (description
-     "JSON library for C++ trying to accomplish “Intuitive syntax”,
-“Trivial integration”, and “Serious testing”.
-However, “Memory efficiency” and “Speed” have not been primary goals.")
-    (license license:expat)))
-
 (define-public python-ruamel.yaml
   (package
     (name "python-ruamel.yaml")

gnu/packages/video.scm

@@ -428,7 +428,7 @@ (define-public mkvtoolnix
        ("gettext" ,gettext-minimal)
        ("googletest" ,googletest)
        ("libxslt" ,libxslt)
-       ("nlohmann-json-cpp" ,nlohmann-json-cpp)
+       ("json-modern-cxx" ,json-modern-cxx)
        ("perl" ,perl)
        ("pkg-config" ,pkg-config)
        ("po4a" ,po4a)