treewide: Make explicit that 'dev' output of libffi is used

pkgs/development/compilers/ecl/default.nix

@@ -29,7 +29,7 @@ stdenv.mkDerivation {
   configureFlags = [
     "--enable-threads"
     "--with-gmp-prefix=${gmp.dev}"
-    "--with-libffi-prefix=${libffi}"
+    "--with-libffi-prefix=${libffi.dev}"
     ]
     ++
     (stdenv.lib.optional (! noUnicode)

pkgs/development/interpreters/pixie/default.nix

@@ -47,7 +47,7 @@ let
     patchPhase = ''
       (cd pixie-src
        patch -p1 < ${./load_paths.patch}
-       libraryPaths='["${libuv}" "${libedit}" "${libffi}" "${boost.dev}" "${boost.out}" "${zlib}"]'
+       libraryPaths='["${libuv}" "${libedit}" "${libffi.dev}" "${boost.dev}" "${boost.out}" "${zlib}"]'
        export libraryPaths
        substituteAllInPlace ./pixie/ffi-infer.pxi)
     '';