Create new appstore scons option for Mac App Store builds.

Disables the reverb effect and vinyl control, which are not available
with the App Store Exception.  Attempting to enable vinylcontrol=1
with appstore=1 will not succeed.
Ensure that Mac App Store build options are explicitly Mac-specific.
Merge pull request #289 from ywwg/mac-disable-reverb

Create new appstore scons option for Mac App Store builds.