In our incremental linking command block we relied on del never
returning a non-zero exit code. Other command line interpreters
than cmd.exe, e.g. TCC, behave differently.
We now check for existence before trying to delete the manifest
file.
Task-number: QTBUG-29698
Change-Id: I94d125998da6f2c0377104f7e83cdd9e8b838329
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>