[TASK] Automatically create _make/conf.py if needed

Custom projects instantiated from a Git repository and based on a
TYPO3 project structure may miss the _make/conf.py file. This is
the case with the example extension manual:
https://git.typo3.org/Documentation/TYPO3/Example/ExtensionManual.git

Change-Id: I5fc493a764080a6146e87fb0aa012ba761aa0146
Related: #57642
Reviewed-on: https://review.typo3.org/29148
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers