I didn't commit a fix to github for this since I'm not sure if adding that function to install.php is the appropriate fix, or if there is another source file the function could be moved to.

Thanks.
has_caching_for was introduced to replace some growingly-complex copy&pasted code. However the installer isn't quite Composr, doesn't use global2.php and doesn't load all the same dependencies. Correct fix was a function_exists guard to battleguard the shared parts of the framework the installer calls up.