Unbuffer so that output from packages appears when redirecting

- Python I/O would not properly interleave (or appear) with output from
  subcommands.

- Add a flusing wrapper around sys.stdout and sys.stderr when
  redirecting, so that Python output is synchronous with that of
  subcommands.

var/spack/repos/builtin.mock/packages/printing-package/package.py

@@ -0,0 +1,45 @@
+##############################################################################
+# 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/llnl/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 PrintingPackage(Package):
+    """This package prints some output from its install method.
+
+    We use this to test whether that output is properly logged.
+    """
+    homepage = "http://www.example.com/printing_package"
+    url      = "http://www.unit-test-should-replace-this-url/trivial_install-1.0.tar.gz"
+
+    version('1.0', 'foobarbaz')
+
+    def install(self, spec, prefix):
+        print("BEFORE INSTALL")
+
+        configure('--prefix=%s' % prefix)
+        make()
+        make('install')
+
+        print("AFTER INSTALL")