Add package for htslib

Htslib was pulled out of samtools and made into a standalone thing.
This commit adds a packag for it, in support of the samtools package.

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

+##############################################################################
+# 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 Htslib(Package):
+    """C library for high-throughput sequencing data formats."""
+
+    homepage = "https://github.com/samtools/htslib"
+    url      = "https://github.com/samtools/htslib/releases/download/1.3.1/htslib-1.3.1.tar.bz2"
+
+    version('1.3.1', '16d78f90b72f29971b042e8da8be6843')
+
+    depends_on('zlib')
+
+    def install(self, spec, prefix):
+        configure('--prefix={0}'.format(prefix))
+        make()
+        make('install')