Building a forked repository doesn't work because of missing tags

Description of documentation issue or suggestion I made a fork of multipass and tried to build it using the instructions in the README.md. This failed on the cmake ../ step because forking the repository doesn't copy tags.

The issue is in the root CMakeLists.txt which contains the following lines

    126   execute_process(COMMAND git describe --long
    127                   WORKING_DIRECTORY ${CMAKE_SOURCE_DIR}
    128                   OUTPUT_VARIABLE GIT_VERSION
    129                   OUTPUT_STRIP_TRAILING_WHITESPACE)
...
    156   string(REGEX MATCH "^v.+-([0-9]+)-(g.+)$" GIT_VERSION_MATCH ${GIT_VERSION})

Since the forked repo has no tags, git describe --long doesn't return anything, and we get an error on line 156:

CMake Error at CMakeLists.txt:156 (string):
  string sub-command REGEX, mode MATCH needs at least 5 arguments total to
  command.

Proposal to fix issue or enact suggestion Forking a repository and trying to build it seems like a common case, so I think it should be possible (without debugging the build process), but there could be two ways to support it:

A simple git fetch --tags https://github.com/canonical/multipass.git seems sufficient to pull the upstream tags into the forked repository, so the "Building" section in README.md could be expanded with something like:

If you're working on a forked repository:
cd <multipass>
git fetch --tags https://github.com/canonical/multipass.git

Alternatively the CMake process could probably be modified to ignore the version number if tags are not available or set it to some dummy value. But I'm not 100% sure since I don't know if it's being used for more than just setting the output of multipass --version.

Additional context I could send a PR for either, but would like to know the maintainers preferred approach would be here.

canonical / multipass

Building a forked repository doesn't work because of missing tags #3476