* cmake: fix handling the case where `git describe` fails
* cmake: fix version recorded in releases
If downloaded as a tarball release, there will be no info from git
to determine the release, so it ends up v0.0.0. If that's the case,
we'll now use the release specified in the project() command,
which needs to be updated for each new release.
* cmake: add `--tags` to `git describe`
That way, lightweight tags will also be taken into account, which should
never hurt, but it'll help in cases where, for some mysterious reason or
other, annotated tags don't make it into a clone.
* update releasing.md
Git was being executed in the current directory, so could not get the
latest tag if cmake was run from a build directory. Force git to be
run from with the source directory.
get_git_version CMake function uses 'git' command directly, instead of
GIT_EXECUTABLE variable. This causes CMake errors while generating
project files in environments, where 'git' command is not present
in PATH.