Backport enhancements from master, commits:

* 9c4bfa66: "make tags": remove -t
  option of ctags. The option was kept for backward compatibility,
  but it was completly removed recently. Patch written by Stéphane
  Wirtel.
* cf0ac6a7: Fix "make tags": set
  locale to C to call sort
* 8a543c0b: `make tags` fixes (GH-717)