Bug 935776: fix bootstrapping on Mac Mavericks

* don't prompt for gcc-4.6 and MacSDK10.6 on Mavericks
* prompt for gnu-tar if no compatible archiver is found
Merge pull request #332 from vicamo/bugzilla/935776/emulator-mac-mavericks

Bug 935776: fix bootstrapping on Mac Mavericks. r=marshall_law