qmake: generate make dist target for subdir projects

The original dist target no longer copies files around, but
merely does the final packaging. It depends on a new recursive
distdir target, which handles copying distfiles to the distdir.

[ChangeLog][Tools][qmake] Added 'make dist' target for subdirs
projects (unix only)

Task-number: QTBUG-21910
Change-Id: Ib59139c3fe196caf832d8dcefab484ab91f1f5ce
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>