configure.sh: prefer 'which' over 'hash'
configure.sh: prefer 'which' over 'hash'

hash may trigger a bashism warning; which is already depended on by this
script

Change-Id: Ibe0e7d1132d72f03aaf5b7d4c6180b59e92aceff

smarter/aom,mwgoldsmith/libvpx,mbebenita/aom,mwgoldsmith/vpx,mwgoldsmith/libvpx,luctrudeau/aom,webmproject/libvpx,mbebenita/aom,Topopiccione/libvpx,mwgoldsmith/libvpx,mwgoldsmith/libvpx,ShiftMediaProject/libvpx,Topopiccione/libvpx,GrokImageCompression/aom,mbebenita/aom,webmproject/libvpx,Topopiccione/libvpx,mwgoldsmith/vpx,luctrudeau/aom,smarter/aom,luctrudeau/aom,ShiftMediaProject/libvpx,smarter/aom,mbebenita/aom,Topopiccione/libvpx,smarter/aom,mbebenita/aom,smarter/aom,webmproject/libvpx,mwgoldsmith/vpx,GrokImageCompression/aom,webmproject/libvpx,mwgoldsmith/libvpx,webmproject/libvpx,luctrudeau/aom,GrokImageCompression/aom,mbebenita/aom,mbebenita/aom,ShiftMediaProject/libvpx,GrokImageCompression/aom,luctrudeau/aom,Topopiccione/libvpx,mbebenita/aom,mbebenita/aom,mwgoldsmith/vpx,ShiftMediaProject/libvpx,mwgoldsmith/libvpx,Topopiccione/libvpx,GrokImageCompression/aom,mwgoldsmith/vpx,webmproject/libvpx,GrokImageCompression/aom,mwgoldsmith/vpx,ShiftMediaProject/libvpx,smarter/aom,luctrudeau/aom