[!25] Add a 2.6 release for the Dorie people

Merge branch 'feature/2.6-release' into 'master'

ref:spack/dune-spack\> This fixes [#17]

TODO:

-   [ ] Adjust README

See merge request [spack/dune-spack!25]

  [#17]: gitlab.dune-project.org/NoneNone/issues/17
  [spack/dune-spack!25]: gitlab.dune-project.org/spack/dune-spack/merge_requests/25


Closes #17

packages/dune/package.py

@@ -38,6 +38,7 @@ class Dune(CMakePackage):
     # the default version) in Python3.
     dune_versions_to_branch = [
       ("2.7", "releases/2.7"),
+      ("2.6", "releases/2.6"),
       ("master" , "master"),
     ]
 
@@ -258,6 +259,13 @@ class Dune(CMakePackage):
         when='@2.7+python',
     )
 
+    resource(
+        name='dune-python',
+        git='https://gitlab.dune-project.org/staging/dune-python.git',
+        branch='releases/2.6',
+        when='@2.6+python',
+    )
+
     # The dune-codegen package does not yet have a 2.7-compatible release
     resource(
         name='dune-codegen',
@@ -266,6 +274,7 @@ class Dune(CMakePackage):
         when='@master+codegen',
         submodules=True,
     )
+    conflicts('dune@2.6', when='+codegen')
     conflicts('dune@2.7', when='+codegen')
 
     # Make sure that Python components integrate well into Spack
@@ -382,7 +391,13 @@ class Dune(CMakePackage):
 
         installer = Executable('bin/dunecontrol')
         options_file = join_path(self.stage.source_path, "..", "dune.opts")
-        installer('--builddir=%s'%self.build_directory ,  '--opts=%s' % options_file, 'cmake')
+
+        # The 'cmake' command of dunecontrol was added in 2.7
+        commandname = 'cmake'
+        if '@2.6' in self.spec:
+            commandname = 'configure'
+
+        installer('--builddir=%s'%self.build_directory ,  '--opts=%s' % options_file, commandname)
 
     def install(self, spec, prefix):
         installer = Executable('bin/dunecontrol')