configure: test for -Wfloat-conversion
configure: test for -Wfloat-conversion

supported by clang, gcc-4.9+

Change-Id: I893766de7307fef9a8b68c0cfae137c9d3b0dbe8

mwgoldsmith/vpx,ShiftMediaProject/libvpx,mwgoldsmith/libvpx,mwgoldsmith/libvpx,mwgoldsmith/vpx,webmproject/libvpx,webmproject/libvpx,mwgoldsmith/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,Topopiccione/libvpx,Topopiccione/libvpx,Topopiccione/libvpx,mwgoldsmith/libvpx,Topopiccione/libvpx,ShiftMediaProject/libvpx,mwgoldsmith/vpx,webmproject/libvpx,ShiftMediaProject/libvpx,mwgoldsmith/libvpx,mwgoldsmith/vpx,webmproject/libvpx,mwgoldsmith/libvpx,ShiftMediaProject/libvpx,Topopiccione/libvpx,webmproject/libvpx,Topopiccione/libvpx,mwgoldsmith/vpx,mwgoldsmith/vpx