Old behaviour was only to accept "Y". The prompt suggested, however,
that "y" was the default behaviour, so accepting "return" as
confirmation of that choice is the sane behaviour with least user
experience disruption.
This fixes#387.
Formerly, `qmake --help` was called, and the return value was checked
for being 0, but `--help` is not a valid option in any of my qmakes.
`-help` would be correct, but even with that, qmake returns a non-zero
value.
So, simply replaced that with a check for the existence of the qmake
executable.
