cmake without git access

Apparently cmake did not work properly without access to the git repository. The reason for that is, that cmake is writing the git revision into a header file, to mark output files with the corresponding git revision. This issue needs further investigations.

Success criteria:

• if git access is available the git revision needs be written into the header
• if git is not available a placeholder needs to be written into the header: e.g. git_not_accessible
• in any case cmake should run properly.