Add Package: nlohmann_json (#7690)

Adds a spack package for the JSON C++ library
  https://github.com/nlohmann/json

var/spack/repos/builtin/packages/adios2/package.py

@@ -77,7 +77,7 @@ class Adios2(CMakePackage):
     # depends_on('googletest')
     # depends_on('pugixml')
     # depends_on('kwsys')
-    # depends_on('nlohmannjson')
+    # depends_on('nlohmann-json')
     # depends_on('pybind11@2.1.1:', when='+python')
 
     depends_on('mpi', when='+mpi')

var/spack/repos/builtin/packages/nlohmann-json/package.py

@@ -0,0 +1,60 @@
+##############################################################################
+# Copyright (c) 2013-2018, Lawrence Livermore National Security, LLC.
+# Produced at the Lawrence Livermore National Laboratory.
+#
+# This file is part of Spack.
+# Created by Todd Gamblin, tgamblin@llnl.gov, All rights reserved.
+# LLNL-CODE-647188
+#
+# For details, see https://github.com/spack/spack
+# Please also see the NOTICE and LICENSE files for our notice and the LGPL.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU Lesser General Public License (as
+# published by the Free Software Foundation) version 2.1, February 1999.
+#
+# This program is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the IMPLIED WARRANTY OF
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the terms and
+# conditions of the GNU Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public
+# License along with this program; if not, write to the Free Software
+# Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
+##############################################################################
+from spack import *
+
+
+class NlohmannJson(CMakePackage):
+    """JSON for Modern C++"""
+
+    homepage = "https://nlohmann.github.io/json/"
+    url      = "https://github.com/nlohmann/json/archive/v3.1.2.tar.gz"
+    maintainers = ['ax3l']
+
+    version('3.1.2', '557651b017c36ad596ba3b577ba1b539')
+
+    variant('single_header', default=True,
+        description='Use amalgamated single-header')
+    variant('test', default=True,
+        description='Build the tests')
+
+    depends_on('cmake@3.8:', type='build')
+
+    # requires mature C++11 implementations
+    conflicts('%gcc@:4.8')
+    conflicts('%gcc@:3.3')
+    conflicts('%intel@:16')
+    conflicts('%pgi@:14')
+
+    def cmake_args(self):
+        spec = self.spec
+
+        args = [
+            '-DJSON_MultipleHeaders:BOOL={0}'.format(
+                'ON' if '~single_header' in spec else 'OFF'),
+            '-DBUILD_TESTING:BOOL={0}'.format(
+                'ON' if '+test' in spec else 'OFF')
+        ]
+
+        return args