Ninja: generator uses relative paths for source files

This issue was created automatically from an original Mantis Issue. Further discussion may take place here.

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This makes it unnecessarily difficult for tools parsing the generated errors to find the referenced file. In particular, jumping to error locations in vim only works when the current directory is the root of the build directory, otherwise the relative paths in the error messages point to the wrong location.

The Unix Makefile generator uses absolute filenames for source files and thus does not suffer from this.

If there's not a strong reason for the current behavior, I would suggest to also use absolute filenames (at least) for source files in the Ninja generator.

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EDIT: See #13894 (comment 947404) for more details.