Libversion

* CMake corrections

* Core algorithm reworked and simplified, which brings many small improvements
* Numbers of arbitrary lengths are now compared correctly
* Versions are now split more uniformly, so now `1.0alpha1` == `1.0.alpha1` == `1.0alpha.1` == `1.0.alpha.1`
* Any number of alphabetic/numeric compoments are now allowed (`1a2b3c4` is no longer effectively trimmed to `1a2`)
* Implement `VERSIONFLAG_LOWER_BOUND` and `VERSIONFLAG_UPPER_BOUND` flags
* Added `version_sort` and `version_explain` (not installed) utilities,
useful for debugging
* Removed deprecated `version_signature` API

* Add support for `errata` keyword as post-release.

* Modernized CMake code, added support for CMake export mechanism, so
it's now super easy to use libversion from CMake
* Added export header so shared library can now be used on Windows
* Better handling of deprecated APIs
* Fixed library name conflicts on Windows

* Fixed undefined behavior in string comparison

* Fixed installation for out-of-source builds
* Documentation updates