qtgui: fixed naming of classes for long-term use.
volk: clang sets GNUC_MINOR only to 2, so we need to check if we're building with clang.

See the attached patch that enables xgetbv for clang >= 3.0.
volk: some versions of clang have problems with cvtpi32_ps.

We make a separate cmake check here for xgetbv and cvtpi32_ps and set definitions accordingly.