mjpegtools: Fix build on POWER targets with AltiVec-free baseline

I'll leave fixing of the AltiVec code to ppc64le users.

pkgs/by-name/mj/mjpegtools/package.nix

@@ -56,6 +56,13 @@ stdenv.mkDerivation rec {
 
   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;