qtgui: fixed problem with new volk avx multiply kernel.

Was calling _a version of kernel, but the window is a std::vector, which is not guaranteed to be 32-byte aligned.
cmake: updates (basically reverts) commit 3c44ba0687b08.

ICE 3.4.2 must be run with GCC 4.6 or less. Versions >= 4.7 will not work. All versions of GCC tested compile ICE 3.5.0/1. This check tests the version of GCC and if it is /NOT/ less than 4.6 (and therefore 4.7 or greater), then issue an error and disable ControlPort.