Bug: $file_ext resolves to file extension + " sign.
I've seen this bug pop up with some batch scripts I wrote a while back. It happened when I took the scripts I had written, and copied them to another computer. Of course, I assumed they would work. They didn't.
As it turns out, the code that windows uses to generate a file name and pass it along to a program is fairly complex, and varies with the version of windows(which as you may not know, is not the same as the edition). In general:
1. if the file path contains spaces, the path is put inside quotes
2. if the file path doesn't contain spaces, the behavior is undefined(just like what a c++ compiler does for any number of ambiguous statements: i++ / 2 - ++i * 5)
From what I've seen, non-english windows tends to enclose filepaths in quotes more often than not. As for why you see it as a single quote at the end of a make variable, seems to me that the parsing for that variable needs a bit more error-checking. Took me 2 days to figure out what was going on with those batch scripts...