identd: Fix "brew audit" problems

identd: Fix "brew audit" problems

* `homepage` (line 6) should be put before `url` (line 4)
* A `test do` test block should be added
* Stable: Redundant :using value in URL
* Formula should have a desc (Description).
* use "xcodebuild *args" instead of "system 'xcodebuild', *args"
* `require "formula"` is now unnecessary
* Use `system "xcodebuild", "-target", "identd", "-configuration", "Release"`
  instead of `system "xcodebuild -target identd -configuration Release"`

Signed-off-by: Alexander Barton <alex@barton.de>