HDF5: is_enabled helper (ON) (#35705)

* HDF5: is_enabled helper (ON)
  Slightly generalize the `is_enabled` helper in the HDF5 package.
  `ON` is the most typical CMake bool option passed, besides many
  other possible `true` values, and should be included as a possible
  check to the config.
* Simplify
---------

Co-authored-by: Tamara Dahlgren <35777542+tldahlgren@users.noreply.github.com>

var/spack/repos/builtin/packages/hdf5/package.py

@@ -489,9 +489,7 @@ def determine_version(cls, exe):
     @classmethod
     def determine_variants(cls, exes, version):
         def is_enabled(text):
-            if text in set(["t", "true", "enabled", "yes", "1"]):
-                return True
-            return False
+            return text.lower() in ["t", "true", "enabled", "yes", "1", "on"]
 
         results = []
         for exe in exes: