Use gtar to support SOURCE_DATE_EPOCH

[cen.is.imba_gmail.com]

Summary

When SOURCE_DATE_EPOCH is used, GNU tar's specific flags are used that BSD tar does not support. Gtar is added as a dep and TAR command overwritten.

Tested a successful build together with patch from https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=291905