treewide: Use targetPrefix instead of prefix for platform name prefixes

Certain tools, e.g. compilers, are customarily prefixed with the name of
their target platform so that multiple builds can be used at once
without clobbering each other on the PATH. I was using identifiers named
`prefix` for this purpose, but that conflicts with the standard use of
`prefix` to mean the directory where something is installed. To avoid
conflict and confusion, I renamed those to `targetPrefix`.

pkgs/development/libraries/ffmpeg/generic.nix

@@ -173,7 +173,7 @@ stdenv.mkDerivation rec {
   crossAttrs = {
     configurePlatforms = [];
     configureFlags = configureFlags ++ [
-      "--cross-prefix=${stdenv.cc.prefix}"
+      "--cross-prefix=${stdenv.cc.targetPrefix}"
       "--enable-cross-compile"
       "--target_os=${hostPlatform.parsed.kernel}"
       "--arch=${hostPlatform.arch}"