Fix version information when not related to a release

[terhorstd]

Revert accidental merge of last release (e396edb).

[clinssen]

Could you comment on how this commit affects #2116?

[jougs]

@terhorst: maybe we should rather put 3.4-pre in that field instead of UNKNOWN? @clinssen: would that help with #2116?

[clinssen]

We could say, if we are based on a branch (if (NEST_VERSION_BRANCH)), then append a "hand-rolled" string to indicate the version it was based on. For instance,

if (NEST_VERSION_BRANCH)
    set(NEST_VERSION_BRANCH "${NEST_VERSION_BRANCH}@3.4-pre")
endif()

results in the version string

#define NEST_VERSION_STRING "master@3.4-pre@71c85a0f8"

Undoubtedly you and Dennis will have a better idea for syntax than my repeated at symbol. We are not presently compliant with https://peps.python.org/pep-0386/; how badly would we want this? An alternative, semantic versioning-compliant string would be

3.4-pre+71c85a0f8

or

3.4-pre+master-71c85a0f8

Updating the hand-rolled string would in any case need to be part of the steps involved in doing a release.