mjpegtools: Fix build on POWER targets with AltiVec-free baseline (#457351)

  postPatch = ''

    sed -i -e '/ARCHFLAGS=/s:=.*:=:' configure

  ''

  # Only ppc64le baseline guarantees AltiVec, no configure option to disable it so just make checks never signal success.

  # AltiVec code also fails without disabling new compiler warnings:

  # quant_non_intra.c:72:42: error: initialization of '__vector unsigned short *' {aka '__vector(8) short unsigned int *'} from incompatible pointer type 'uint16_t *' {aka 'short unsigned int *'} [-Wincompatible-pointer-types]

  + lib.optionalString (!(stdenv.hostPlatform.isPower64 && stdenv.hostPlatform.isLittleEndian)) ''

    substituteInPlace configure \

      --replace-fail 'have_altivec=true' 'have_altivec=false'

  '';

  enableParallelBuilding = true;