ZeroTierOne/ext/tap-mac/tuntap

Building the tap for both x86_64 and i386 requires an older version of the
Xcode tools than what now ships for Mavericks (10.9). The newer version
does not support creating i386 kernel images.

At the moment this is done on an OSX 10.6 virtual image that is used for
building. (It doesn't have to be done often.) Then the kext is signed on
the regular build system. That's because images built on newer OSX don't
seem to load on 10.6 but 10.6 built kexts seem fine on 10.9. Go figure.

Older Xcode can also be found at:

https://developer.apple.com/downloads

It requires a bit of a dance to unpack the package and obtain an unpacked
tree, but once it's there you can change the line in tap/Makefile and
build for both architectures.

This will go on until i386 is thoroughly legacy, at which point we'll
probably start just supporting x86_64. But that might be a while. We want
to support old Macs through their entire useful life.

Since this build is irritating, a pre-built copy is packaged in ext/ and
is installed by 'make install'. So users shouldn't have to build this.