glu: new virtual package; openglu: new package (#7162)

* mesa-glu: now provides 'glu' (GL Utility library)

* openglu: add package, provides glu

* openglu: add custom fetcher, remove nonsense url

Change openglu package to look like the opengl package in GitHub PR

* [OpenGLU] Fix a typo

var/spack/repos/builtin/packages/mesa-glu/package.py

@@ -37,3 +37,5 @@ class MesaGlu(AutotoolsPackage):
     variant('mesa', default=True,
        description='Usually depends on mesa, disable for accelerated OpenGL')
     depends_on('mesa', when='+mesa')
+
+    provides('glu@1.3')