make stubs for dangerous/useless qmake functions in limited mode

instead of having them "not implemented" (and consequently getting
errors), just let requires(), system(), mkpath(), write_file(), touch()
and cache() do nothing. this is likely to cause followup failures which
are harder to detect, but the amount of scary noise we are producing now
is not really acceptable. furthermore, in qtcreator these failures
actually terminate the "precise" evaluation, which breaks Run
Configurations.

Task-number: QTBUG-28159
Task-number: QTCREATORBUG-8550 (in different repo)
Change-Id: I1bdeb759e895e4200f09332dadf8a6cef348182f
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>