We currently do not set up a project version within CMake, meaning that
it can't be use by other projects including libgit2 as a sub-project and
also not by other tools like IDEs.

This commit changes this to always set up a project version, but instead
of extracting it from the "version.h" header we now set it up directly.
This is mostly to avoid mis-use of the previous `LIBGIT2_VERSION`
variables, as we should now always use the `libgit2_VERSION` ones that
are set up by CMake if one provides the "VERSION" keyword to the
`project()` call. While this is one more moving target we need to adjust
on releases, this commit also adjusts our release script to verify that
the project version was incremented as expected.