Ensure `tar` is available for native extension compilation

Many gems ship with native extensions that assume tar will be available in the PATH during the build process. Although we would hope these native extensions respect environment variables like `TAR` or `PROG_TAR` it’s not always the case. :-(

This issue was uncovered while trying to compile [dep-selector-libgecode](https://github.com/opscode/dep-selector-libgecode) as part of a ChefDK build  [BEFORE the chef software definition copies `bsdtar.exe` over as `tar.exe`](https://github.com/opscode/omnibus-chef/blob/release/chef-11.16.0/config/software/chef-windows.rb#L45-L60). Although we previously thought setting the `PROG_TAR` environment variable to `bsdtar.exe` fixed the issue (see opscode/dep-selector-libgecode#33 and opscode/omnibus-software#298) there are [parts of the Makefile which don't respect `PROG_TAR` and hardcode `tar`](https://github.com/opscode/dep-selector-libgecode/blob/4aab3b7f1390450475ebe94186f0ecb3c282d663/ext/libgecode3/vendor/gecode-3.7.3/Makefile.in#L1499-L1502).

/cc @opscode/release-engineers @opscode/client-engineers