Treat paths which are relative and which contain a generator
expression which is not at the beginning as relative to the
source directory.
This matches the behavior of paths which are relative but contain
no generator expression at all.
Previously this would generate a relative path with the IMPORTED
target on export(), which would be a reported as a non-existent
path on import. If used directly in the buildsystem, it would be
reported as a relative path, which is also an error. There is no
need for a policy in this case.
38afc82e76