Merge branch 'mplegendre-multi_pkgsrc_roots' into develop

- This moves var/spack/packages to var/spack/repos/builtin/packages.

- Packages that did not exist in the source branch, or were changed in
  develop, were moved into var/spack/repos/builtin/packages as part of
  the integration.

Conflicts:
	lib/spack/spack/test/unit_install.py
	var/spack/repos/builtin/packages/clang/package.py

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

@@ -0,0 +1,13 @@
+from spack import *
+
+class Tar(Package):
+    """GNU Tar provides the ability to create tar archives, as well as various other kinds of manipulation."""
+    homepage = "https://www.gnu.org/software/tar/"
+    url      = "http://ftp.gnu.org/gnu/tar/tar-1.28.tar.gz"
+
+    version('1.28', '6ea3dbea1f2b0409b234048e021a9fd7')
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix)
+        make()
+        make('install')