Proper versioning

[pablode]

guc has a version defined in the top-level CMakeLists.txt, but it is not propagated to the library and application. We should

• introduce a --version flag to the executable
• #define the major, minor and patch version numbers in the libguc header
• write the exact guc version string to the asset Documentation metadatum

Furthermore, it might make sense to include the commit hash in non-release version strings.