In MakefileGenerator::initOutPaths() we ensure that directory
variables end with a directory separator, except for DLLDESTDIR.
There doesn't seem to be a valid reason for this exception.
Remove it for the sake of simplifying the code base.
Change-Id: I60eb01b410161e6e1d147d76f044f5140a7573bd
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>