Make frontend OS on Cray machines a proper linux distro.

lib/spack/spack/platforms/cray_xc.py

@@ -10,7 +10,6 @@ class CrayXc(Platform):
     back_end    = 'ivybridge'
     default     = 'ivybridge'
 
-    front_os    = "SuSE11"
     back_os     = "CNL10"
     default_os  = "CNL10"
 
@@ -38,7 +37,10 @@ def __init__(self):
         self.add_target('haswell',
                         Target('haswell','craype-haswell'))
 
-        self.add_operating_system('SuSE11', LinuxDistro())
+        # Front end of the cray platform is a linux distro.
+        linux_dist = LinuxDistro()
+        self.front_os = str(linux_dist)
+        self.add_operating_system(str(linux_dist), linux_dist)
         self.add_operating_system('CNL10', Cnl())
 
     @classmethod
@@ -50,4 +52,3 @@ def detect(self):
             if '-D__CRAYXC' in text:
                 return True
         return False
-