darwin: Only enable clang cflags if we use clang
darwin: Only enable clang cflags if we use clang

i.e. only on osx 10.8 and 10.9

ramaxlo/cerbero,flexVDI/cerbero,superdump/cerbero,AlertMe/cerbero,nzjrs/cerbero,nirbheek/cerbero,superdump/cerbero,lubosz/cerbero,GStreamer/cerbero,superdump/cerbero,ford-prefect/cerbero,sdroege/cerbero,atsushieno/cerbero,ikonst/cerbero,lubosz/cerbero,fluendo/cerbero,nzjrs/cerbero,freedesktop-unofficial-mirror/gstreamer__sdk__cerbero,EricssonResearch/cerbero,GStreamer/cerbero,EricssonResearch/cerbero,ford-prefect/cerbero,jackjansen/cerbero,nzjrs/cerbero,shoreflyer/cerbero,shoreflyer/cerbero,freedesktop-unofficial-mirror/gstreamer__sdk__cerbero,nirbheek/cerbero-old,centricular/cerbero,justinjoy/cerbero,freedesktop-unofficial-mirror/gstreamer__sdk__cerbero,ikonst/cerbero,AlertMe/cerbero,nirbheek/cerbero-old,sdroege/cerbero,brion/cerbero,centricular/cerbero,ford-prefect/cerbero,multipath-rtp/cerbero,fluendo/cerbero,flexVDI/cerbero,ramaxlo/cerbero,nirbheek/cerbero,atsushieno/cerbero,nirbheek/cerbero,multipath-rtp/cerbero,EricssonResearch/cerbero,nzjrs/cerbero,ikonst/cerbero,EricssonResearch/cerbero,ramaxlo/cerbero,shoreflyer/cerbero,brion/cerbero,fluendo/cerbero,ikonst/cerbero,brion/cerbero,jackjansen/cerbero,GStreamer/cerbero,shoreflyer/cerbero,ford-prefect/cerbero,AlertMe/cerbero,brion/cerbero,freedesktop-unofficial-mirror/gstreamer__cerbero,ramaxlo/cerbero,sdroege/cerbero,nzjrs/cerbero,freedesktop-unofficial-mirror/gstreamer__sdk__cerbero,multipath-rtp/cerbero,flexVDI/cerbero,freedesktop-unofficial-mirror/gstreamer__cerbero,centricular/cerbero,shoreflyer/cerbero,sdroege/cerbero,lubosz/cerbero,justinjoy/cerbero,OptoFidelity/cerbero,justinjoy/cerbero,jackjansen/cerbero,multipath-rtp/cerbero,AlertMe/cerbero,OptoFidelity/cerbero,atsushieno/cerbero,atsushieno/cerbero,lubosz/cerbero,superdump/cerbero,atsushieno/cerbero,fluendo/cerbero,fluendo/cerbero,centricular/cerbero,nirbheek/cerbero-old,flexVDI/cerbero,ikonst/cerbero,nirbheek/cerbero,AlertMe/cerbero,nirbheek/cerbero-old,OptoFidelity/cerbero,justinjoy/cerbero,freedesktop-unofficial-mirror/gstreamer__cerbero,multipath-rtp/cerbero,sdroege/cerbero,GStreamer/cerbero,freedesktop-unofficial-mirror/gstreamer__cerbero,OptoFidelity/cerbero,EricssonResearch/cerbero,ramaxlo/cerbero,centricular/cerbero,jackjansen/cerbero,brion/cerbero,GStreamer/cerbero,flexVDI/cerbero