nlohmann-json: v3.7.0 (#12517)

* nlohmann-json: 3.7.0

The version 3.6.0 fix compile issues with GCC 9.0.

* nlohmann-json: proper test handling

tests are controlled via `spack install --test root|all ...`
instead of a variant.

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

@@ -13,6 +13,7 @@ class NlohmannJson(CMakePackage):
     url      = "https://github.com/nlohmann/json/archive/v3.1.2.tar.gz"
     maintainers = ['ax3l']
 
+    version('3.7.0', sha256='d51a3a8d3efbb1139d7608e28782ea9efea7e7933157e8ff8184901efd8ee760')
     version('3.5.0', sha256='e0b1fc6cc6ca05706cce99118a87aca5248bd9db3113e703023d23f044995c1d')
     version('3.4.0', sha256='c377963a95989270c943d522bfefe7b889ef5ed0e1e15d535fd6f6f16ed70732')
     version('3.3.0', sha256='2fd1d207b4669a7843296c41d3b6ac5b23d00dec48dba507ba051d14564aa801')
@@ -22,8 +23,6 @@ class NlohmannJson(CMakePackage):
 
     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')
 
@@ -42,7 +41,7 @@ def cmake_args(self):
             '-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')
+                'ON' if self.run_tests else 'OFF')
         ]
 
         return args