configure: check for arm_neon.h w/neon builds
configure: check for arm_neon.h w/neon builds

fails at configure time rather than compile time unless using
--enable-external-build

Change-Id: I966ee1000e28fdcc3f4a29759789b056faee0010

webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx