libexpat: Fix for older systems, move to GitHub (#8135)

* PR #4945 did not make this work on SuSE 11.  Adding the latest version of libexpat did.  Also update for Expat's move to GitHub.

* Update package.py

* Update package.py

Move to url_for_version()

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

@@ -29,8 +29,8 @@
 class Expat(AutotoolsPackage):
     """Expat is an XML parser library written in C."""
 
-    homepage = "http://expat.sourceforge.net/"
-    url      = "https://sourceforge.net/projects/expat/files/expat/2.2.2/expat-2.2.2.tar.bz2"
+    homepage = "https://libexpat.github.io/"
+    url      = "https://github.com/libexpat/libexpat/releases/download/R_2_2_2/expat-2.2.2.tar.bz2"
 
     # Version 2.2.2 introduced a requirement for a high quality
     # entropy source.  "Older" linux systems (aka CentOS 7) do not
@@ -44,9 +44,14 @@ class Expat(AutotoolsPackage):
             description="Use libbsd (for high quality randomness)")
     depends_on('libbsd', when="@2.2.1:+libbsd")
 
+    version('2.2.5', '789e297f547980fc9ecc036f9a070d49')
     version('2.2.2', '1ede9a41223c78528b8c5d23e69a2667')
     version('2.2.0', '2f47841c829facb346eb6e3fab5212e2')
 
+    def url_for_version(self, version):
+        url = 'https://github.com/libexpat/libexpat/releases/download/R_{0}/expat-{1}.tar.bz2'
+        return url.format(version.underscored, version.dotted)
+
     def configure_args(self):
         spec = self.spec
         args = []