Does nothing if no prexisting config.guess is found. (#2446)

* Does nothing if no prexisting config.guess is found.

* Update name for RHEL OS.

lib/spack/spack/build_systems/autotools.py

@@ -80,6 +80,8 @@ def do_patch_config_guess(self):
                 return True
             except:
                 pass
+        else:
+            return True
 
         # Look for a spack-installed automake package
         if 'automake' in self.spec:
@@ -120,7 +122,7 @@ def patch(self):
         """Perform any required patches."""
 
         if self.patch_config_guess and self.spec.satisfies(
-                'arch=linux-redhat7-ppc64le'):
+                'arch=linux-rhel7-ppc64le'):
             if not self.do_patch_config_guess():
                 raise RuntimeError('Failed to find suitable config.guess')