mkspec for macx-icc missing deployment variable

I think that the qmake.conf file is missing the following variable assignment

QMAKE_MACOSX_DEPLOYMENT_TARGET = 10.6

That causes problems when building my project with the intel compiler

This variable is in the macx-clang/qmake.conf file so I think that it should be in the qmake.conf file for the intel build.

When I add this line my project builds as I expected