RFE: add a qmake portgroup

[cooljeanius (Eric Gallager)]

qmake currently has multiple issues that ports have to work around (as brought up in tickets #38272 and #38250):

• qmake isn't ​UsingTheRightCompiler (cf. comment:ticket:38250:1)
• qmake doesn't handle -arch flags properly (cf. comment:ticket:38250:2)
• qmake doesn't handle the --disable-dependency-tracking flag properly (cf. comment:ticket:38272:1)

And I'm sure there's other things that could be done with a qmake portgroup, too.