When discovering tools, configure checks if it is a program that can handle unix-style paths or not. If it isn't, a fixpath prefix is automatically added.

But when the user supplies a tool like `configure FOO=foo.exe`, this does not happen. If that tool does not understand unix-style paths, then it will fail, and there is no easy way for the user to fix this.

Instead, we should apply the fixpath check also to user-supplied tools.