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/pcre2/package.py

@@ -0,0 +1,15 @@
+from spack import *
+
+class Pcre2(Package):
+    """The PCRE2 package contains Perl Compatible Regular Expression
+       libraries. These are useful for implementing regular expression
+       pattern matching using the same syntax and semantics as Perl 5."""
+    homepage = "http://www.pcre.org"""
+    url      = "ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre/pcre2-10.20.tar.bz2"
+
+    version('10.20', 'dcd027c57ecfdc8a6c3af9d0acf5e3f7')
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix)
+        make()
+        make("install")