new package virtualgl (#4297)

* new package virtualgl, depends on mesa-glu but NOT on mesa, so added a variant to mesa-glu

* add comment on mesa variant of mesa-glu

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

@@ -34,4 +34,6 @@ class MesaGlu(AutotoolsPackage):
 
     version('9.0.0', 'bbc57d4fe3bd3fb095bdbef6fcb977c4')
 
-    depends_on('mesa')
+    variant('mesa', default=True,
+       description='Usually depends on mesa, disable for accelerated OpenGL')
+    depends_on('mesa', when='+mesa')