Libversion

* Handle any keyword which starts with `post` equally (as post-release). This
is now also consistent with how keywords starting with `pre` are handled.

* Added special handling of `post` keyword (as post-release, similar to `patch`)

* Add `version_signature_single()` function to generate format signature
for a version string
* Header file renamed to `libversion/version.h`

* Add `version_compare_flags()`, equivalent of `version_compare_simple()`
which takes additional `flags` argument used to fine tune its
behavior.
* Implemented `VERSIONFLAG_P_IS_PATCH` flag which affects the handling
of `p` letter encountered in versions.

* Typing and style fixes

* Honor -D/-U arguments from pkg-config