Local build is not working on OSX

[rappizs]

mage build returns an error related to the OSX date: date: illegal option -- d

The bug was introduced here e23ad22, and the problem is related to the mage.GenerateLDFlags (https://github.com/kubernetes-sigs/release-utils/blob/1bf6b4c5d954/mage/version.go#L73) function. This func runs the date command but this works differently on Linux and on OSX (Unix).

For more info please take a look on this topic: https://stackoverflow.com/questions/9804966/date-command-does-not-follow-linux-specifications-mac-os-x-lion

The quick workaround is to install the GNU based date on Mac, brew install coreutils, and then link your date to point to gdate. It is not documented at the moment and I would suggest to use a build process that doesn't require to change system level programs.

[rappizs]

Created upstream issue: kubernetes-sigs/release-utils#90