Add GCC 7 (#4144)

* Add GCC 7
* Allow users to build subset of GCC compilers with multi-value variant.
* Add comment explaining what 'all' actually means
* More specific golang support
* Only require Zip when building Java
* Comment out provide directives that don't currently work
* Add Ada support
* Use conflicts directive
* Fix joining of languages
* Need special flag to build jit
* Explicitly declare GNAT download extension
* Import tty, update lib64 to lib
* BRIG and Go are not supported on macOS
* Simplify formatting and imports
* JIT patch required for newer versions as well

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

@@ -0,0 +1,51 @@
+##############################################################################
+# 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 Gnat(MakefilePackage):
+    """The GNAT Ada compiler. Ada is a modern programming language designed
+    for large, long-lived applications - and embedded systems in particular
+    - where reliability and efficiency are essential."""
+
+    homepage = "https://libre.adacore.com/tools/gnat-gpl-edition/"
+
+    # NOTE: This is a binary installer intended to bootstrap GCC's Ada compiler
+
+    # There may actually be a way to install GNAT from source. If you go to
+    # the GNAT Download page: https://libre.adacore.com/download/
+    # select "Free Software or Academic Development", select your platform,
+    # expand GNAT Ada, and expand Sources, you'll see links to download the
+    # source code for GNAT and all of its dependencies. Most of these
+    # dependencies are already in Spack.
+
+    # This is the GPL release for Linux x86-64
+    version('2016', '9741107cca1a6a4ddb0d5e8de824a90c', extension='tar.gz',
+            url="http://mirrors.cdn.adacore.com/art/5739cefdc7a447658e0b016b")
+
+    phases = ['install']
+
+    def install(self, spec, prefix):
+        make('ins-all', 'prefix={0}'.format(prefix))