Update gl/qt options for vtk package (#6551)

This provides options for hardware and software rendering with the
gl API and updates the vtk package to make use of those options:

* Create new "gl" virtual package, provided by mesa
* Add external-only "opengl" package, which represents a system
  install that provides gl
* For vtk: prefer system gl implementation by default to get speed
  up from hardware rendering. When software rendering is specifically
  requested (+osmesa), try to use the llvmpipe approach, as it is
  much faster.
* Make qt dependency optional for vtk
* Add basic support for VTK on osx
* qt: Depend on virtual gl package, rather than mesa impl

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

@@ -0,0 +1,58 @@
+##############################################################################
+# Copyright (c) 2013-2017, 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/spack/spack
+# Please also see the NOTICE and LICENSE files 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 Opengl(Package):
+    """Placeholder for external OpenGL libraries from hardware vendors"""
+
+    homepage = "https://www.opengl.org/"
+    url      = "https://www.opengl.org/"
+
+    version('3.2', 'N/A')
+
+    provides('gl@:4.5', when='@4.5:')
+    provides('gl@:4.4', when='@4.4:')
+    provides('gl@:4.3', when='@4.3:')
+
+    def install(self, spec, prefix):
+        msg = """This package is intended to be a placeholder for
+        system-provided OpenGL libraries from hardware vendors.  Please
+        download and install OpenGL drivers/libraries for your graphics
+        hardware separately, and then set that up as an external package.
+        An example of a working packages.yaml:
+
+        packages:
+          opengl:
+            paths:
+              opengl@4.5.0: /opt/opengl
+            buildable: False
+
+        In that case, /opt/opengl/ should contain these two folders:
+
+        include/GL/       (opengl headers, including "gl.h")
+        lib               (opengl libraries, including "libGL.so")"""
+
+        raise InstallError(msg)