Check for bswap* builtins before using
Check for bswap* builtins before using

Canonical builtin checks for clang are to use
__has_builtin. Much less fragile than version checks.

https://code.google.com/p/webm/issues/detail?id=1082

Change-Id: I8151fb75899acdf1a935c23aad9441da99a9abcd

mbebenita/aom,ittiamvpx/libvpx-1,webmproject/libvpx,openpeer/libvpx_new,jmvalin/aom,mbebenita/aom,webmproject/libvpx,luctrudeau/aom,openpeer/libvpx_new,mwgoldsmith/vpx,thdav/aom,jmvalin/aom,webmproject/libvpx,ShiftMediaProject/libvpx,luctrudeau/aom,mbebenita/aom,shacklettbp/aom,smarter/aom,webmproject/libvpx,ShiftMediaProject/libvpx,mbebenita/aom,Topopiccione/libvpx,mwgoldsmith/vpx,webmproject/libvpx,ittiamvpx/libvpx-1,luctrudeau/aom,luctrudeau/aom,smarter/aom,mwgoldsmith/libvpx,jmvalin/aom,openpeer/libvpx_new,luctrudeau/aom,Topopiccione/libvpx,ShiftMediaProject/libvpx,shacklettbp/aom,jmvalin/aom,shacklettbp/aom,openpeer/libvpx_new,openpeer/libvpx_new,openpeer/libvpx_new,smarter/aom,GrokImageCompression/aom,smarter/aom,Topopiccione/libvpx,ittiamvpx/libvpx-1,thdav/aom,Topopiccione/libvpx,mbebenita/aom,shacklettbp/aom,GrokImageCompression/aom,shacklettbp/aom,GrokImageCompression/aom,mwgoldsmith/libvpx,mwgoldsmith/vpx,mwgoldsmith/libvpx,thdav/aom,jmvalin/aom,thdav/aom,mbebenita/aom,webmproject/libvpx,Topopiccione/libvpx,mwgoldsmith/vpx,mwgoldsmith/vpx,jmvalin/aom,mwgoldsmith/vpx,mwgoldsmith/libvpx,smarter/aom,smarter/aom,ShiftMediaProject/libvpx,shacklettbp/aom,ShiftMediaProject/libvpx,mwgoldsmith/libvpx,GrokImageCompression/aom,luctrudeau/aom,mbebenita/aom,GrokImageCompression/aom,thdav/aom,mwgoldsmith/libvpx,ittiamvpx/libvpx-1,Topopiccione/libvpx,ittiamvpx/libvpx-1,mbebenita/aom,GrokImageCompression/aom,thdav/aom,ittiamvpx/libvpx-1,mbebenita/aom