Add new target 'make dist-osx' to create a .pkg installer for OS X
Make OS X installer build from /tmp/dist/pkgroot, and have it be part of the 'make dist' target
auto merge of #12546 : bleibig/rust/pkg-osx, r=brson

This adds a new make target 'dist-osx' to create a .pkg installer for OS X, working toward #9876. This of course only works on OS X.