PNGwriter: Add Spack Package

Adds a package for
[PNGwriter](https://github.com/pngwriter/pngwriter/),
a simple high-level C++ png API used in scientific projects.

```
PNGwriter is a very easy to use open source graphics library that
uses PNG as its output format. The interface has been designed to be
as simple and intuitive as possible. It supports plotting and reading
pixels in the RGB (red, green, blue), HSV (hue, saturation,
value/brightness) and CMYK (cyan, magenta, yellow, black) colour
spaces, basic shapes, scaling, bilinear interpolation, full TrueType
antialiased and rotated text support, bezier curves, opening existing
PNG images and more.
```

PNGwriter is a dependency for [PIConGPU](http://picongpu.hzdr.de),
an open-source, many-core, fully-relativistic particle-in-cell code
and further software developed at
[Helmholz-Zentrum Dresden - Rossendorf](https://www.hzdr.de).

var/spack/repos/builtin/packages/pngwriter/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 Pngwriter(Package):
+    """
+    PNGwriter is a very easy to use open source graphics library that uses PNG
+    as its output format. The interface has been designed to be as simple and
+    intuitive as possible. It supports plotting and reading pixels in the RGB
+    (red, green, blue), HSV (hue, saturation, value/brightness) and CMYK (cyan,
+    magenta, yellow, black) colour spaces, basic shapes, scaling, bilinear
+    interpolation, full TrueType antialiased and rotated text support, bezier
+    curves, opening existing PNG images and more.
+    """
+
+    homepage = "http://pngwriter.sourceforge.net/"
+    url      = "https://github.com/pngwriter/pngwriter/archive/0.5.6.tar.gz"
+
+    version('0.5.6', 'c13bd1fdc0e331a246e6127b5f262136')
+
+    depends_on('cmake', type='build')
+    depends_on('libpng')
+    depends_on('zlib')
+    depends_on('freetype')
+
+    def install(self, spec, prefix):
+        with working_dir('spack-build', create=True):
+            cmake('-DCMAKE_INSTALL_PREFIX=%s' % prefix,
+                  '..', *std_cmake_args)
+
+            make()
+            make('install')