Libversion

* Deprecated `version_compare3()` as well, as it would create ambiguities
for higher level language wrappers
* Exposed libversion version to consumer code via `LIBVERSION_VERSION_ATLEAST`
macro

* API cleanup: all `version_compare*` methods are now replaced with
`version_compare2()`, `version_compare3()`, and `version_compare4()`
which are consistent and suggest the number of arguments.
`version_compare_simple()`, `version_compare_flags()`, and
`version_compare_flags2()` are thus deprecated and will be removed in
further releases.
* Also deprecate `version_signature_simple()` which had no practical use.

* Implemented `version_compare_flags2()` 4-argument function which allows
to specify flags for either compared version independently, without
the need to squish them into the single argument
* Added shared library

* Implemented `VERSIONFLAG_ANY_IS_PATCH` flag which makes any alphabetic
characters encountered in versions treated as post-release.

* Allow to specify `P_IS_PATCH` flag to each comparison argument separately

* Handle `pl` ("patchlevel") keyword as post-release.