declare mpi variant in PyNN package

packages/py-pynn/package.py

@@ -23,6 +23,8 @@ class PyPynn(PythonPackage):
     version('0.8.1', sha256='ce94246284588414d1570c1d5d697805f781384e771816727c830b01ee30fe39')
     version('0.7.5', sha256='15f75f422f3b71c6129ecef23f29d8baeb3ed6502e7a321b8a2596c78ef7e03c')
 
+    variant('mpi', default=False, description='Enable MPI support')
+
     depends_on('python@2.6:2.8,3.3:')
     depends_on('py-jinja2@2.7:',        type=('build', 'run'))
     depends_on('py-docutils@0.10:',     type=('build', 'run'))
@@ -34,6 +36,9 @@ class PyPynn(PythonPackage):
     depends_on('py-neo@0.5.0:',         type=('build', 'run'), when="@0.9.0:")
     depends_on('py-lazyarray',          type=('build', 'run'), when="@0.9.0:")
 
+    depends_on('mpi', when='+mpi')
+    depends_on('py-mpi4py', type=('build', 'run'), when='+mpi')
+
     depends_on('py-mock@1.0:', type='test')
 
     patch('pynn-0.9.6-python3.patch', when='@0.9.6 ^python@3:')