Add option to generate an include wrapper for files that do not yet exist

[jphickey]

Is your feature request related to a problem? Please describe.
A side effect of a recent change to generate_config_includefile is that if the name specified FALLBACK_FILE option does not exist, it now reports that as an error, whereas previously it would create a wrapper to the (nonexistent) file.

The error is correct for most use cases, but if the underlying file is also generated, then there is an initial condition issue when running the build for the first time from a clean slate.

Describe the solution you'd like
If the source for generate_config_includefile is in turn also a generated file, should blindly create the wrapper and not check for existence. It shouldn't even do the normal search in this case, as it would be incorrect to override a generated file with a static one.

Requester Info
Joseph Hickey, Vantage Systems, Inc.