1.6.1 - libtool `-release` flag no longer used

⚠️ NOTE: This release contains no code changes. If you have already upgraded to 1.6.0, there is no need to upgrade to 1.6.1 for other reasons than avoiding future unnecessary rebuilds of reverse dependencies.

This release marks the end of including the MAJOR.MINOR API version fields in the library names. The abi fields have also been cleared, since they weren't that useful when used in conjunction with the libtool -release flag.

---

Actual runtime compatibility (for reference)

Where a --> b means that a is backwards abi-compatible with b:

1.6.1 --> 1.6.0 --> 1.5.1 --> 1.5.0 --> 1.3.0

1.4.0 is not abi-compatible with any other release.