FeatureSummary: Allow to combine and reorder values for the "WHAT" option

For example it is now possible to do:

  feature_summary(WHAT ENABLED_FEATURES
                       DISABLED_FEATURES)

That produces an output like:

  --
  -- The following features have been enabled:

   * Feature1
   * Feature2

  -- The following features have been disabled:
   * Feature2

Skipping the lists that are empty.

Without this patch this would require to do:

  message(STATUS "")
  feature_summary(WHAT ENABLED_FEATURES
      DESCRIPTION "The following features have been enabled:")
  feature_summary(WHAT DISABLED_FEATURES
      DESCRIPTION "The following features have been disabled:")

And empty lists would not be skipped.

The behaviour when WHAT has a single argument (ALL or any option) is
unchanged.
FeatureSummary: Add unit tests
Merge topic 'FeatureSummary_combine_WHAT_values' into next

8372b455 FeatureSummary: Add unit tests
d348cd1b FeatureSummary: Allow to combine and reorder values for the "WHAT" option