New package: libnetworkit (#15714)

* New package libnetworkit

* Fixed artificial dependency restriction

var/spack/repos/builtin/packages/libnetworkit/0001-Name-agnostic-import-of-tlx-library.patch

+--- a/CMakeLists.txt
++++ b/CMakeLists.txt
+@@ -203,10 +203,11 @@ if(NOT NETWORKIT_EXT_TLX)
+ 				"Please run `git submodule update --init` to fetch the submodule.")
+ 	endif()
+ else()
++	file(GLOB tlx_path ${NETWORKIT_EXT_TLX}/lib/**.a)
+ 	add_library(tlx STATIC IMPORTED)
+ 	set_target_properties(tlx PROPERTIES
+-			IMPORTED_LOCATION "${NETWORKIT_EXT_TLX}/lib/libtlx.a"
+-			INTERFACE_INCLUDE_DIRECTORIES "${NETWORKIT_EXT_TLX}/include/")
++		IMPORTED_LOCATION "${tlx_path}"
++		INTERFACE_INCLUDE_DIRECTORIES "${NETWORKIT_EXT_TLX}/include/")
+ endif()
+ 
+ ################################################################################
+

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

+# Copyright 2013-2020 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+from spack import *
+
+
+class Libnetworkit(CMakePackage):
+    """NetworKit is a growing open-source toolkit for large-scale network
+       analysis. Its aim is to provide tools for the analysis of large networks
+       in the size range from thousands to billions of edges. For this purpose,
+       it implements efficient graph algorithms, many of them parallel to
+       utilize multicore architectures. These are meant to compute standard
+       measures of network analysis, such as degree sequences, clustering
+       coefficients, and centrality measures. In this respect, NetworKit is
+       comparable to packages such as NetworkX, albeit with a focus on
+       parallelism and scalability."""
+
+    homepage = "https://networkit.github.io/"
+    url      = "https://github.com/networkit/networkit/archive/6.1.tar.gz"
+
+    maintainers = ['fabratu']
+
+    version('6.1', sha256='22c953ea1054c356663b31c77114c2f0c8fec17e0e707aeec23026241beab9b2')
+
+    variant('static', default=False, description='Enables the build of shared libraries')
+    variant('doc', default=False, description='Enables the build with sphinx documentation')
+
+    depends_on('libtlx')
+    depends_on('py-sphinx', when='+doc', type='build')
+
+    patch('0001-Name-agnostic-import-of-tlx-library.patch', when='@6.1')
+
+    def cmake_args(self):
+        spec = self.spec
+
+        tlx_libs = spec['libtlx'].prefix
+
+        args = ['-DNETWORKIT_EXT_TLX=%s' % tlx_libs,
+                '-DNETWORKIT_STATIC=%s' %
+                ('ON' if '+static' in spec else 'OFF')]
+
+        return args