Add bertini package

Also add missing dependency. Flex would not build for me without bison.
It complained about not being able to find `yacc`.

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

@@ -0,0 +1,49 @@
+##############################################################################
+# Copyright (c) 2013-2016, 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/llnl/spack
+# Please also see the LICENSE file 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 Bertini(Package):
+    """Bertini is a general-purpose solver, written in C, that was created
+    for research about polynomial continuation. It solves for the numerical
+    solution of systems of polynomial equations using homotopy continuation."""
+
+    homepage = "https://bertini.nd.edu/"
+    url      = "https://bertini.nd.edu/BertiniSource_v1.5.tar.gz"
+
+    version('1.5', 'e3f6cc6e7f9a0cf1d73185e8671af707')
+
+    variant('mpi', default=True, description='Compile in parallel')
+
+    depends_on('flex')
+    depends_on('bison')
+    depends_on('gmp')
+    depends_on('mpfr')
+    depends_on('mpi', when='+mpi')
+
+    def install(self, spec, prefix):
+        configure('--prefix=%s' % prefix)
+
+        make()
+        make("install")

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

@@ -33,6 +33,8 @@ class Flex(Package):
     version('2.6.0', '5724bcffed4ebe39e9b55a9be80859ec')
     version('2.5.39', 'e133e9ead8ec0a58d81166b461244fde')
 
+    depends_on('bison')
+
     def install(self, spec, prefix):
         configure("--prefix=%s" % prefix)