* Fix Mac platform check for dependency in py-ipython package: 'when'
constraints in Spack directives must be Specs (either a Spec
object or a Spec in string format)
* Fix Mac version check in py-numpy: platform.mac_ver() returns a
3-part string as its first tuple item so the check as written would
never pass; use Spack Version object to simplify check.
* Fix Mac version check in qt package (the check was incorrectly
comparing ints and strings) and use Spack version object to
simplify check.
9af155f0f6