icu: fix configure to use ${stdenv.shell}

On darwin, not doing this means the configure script uses a hard-coded
/bin/sh for its echo tests, resulting in ECHO_N and ECHO_C settings in
icu-config which are incorrect for the stdenv.shell which it is actually
uses to run.

pkgs/development/libraries/icu/default.nix

@@ -26,6 +26,10 @@ stdenv.mkDerivation {
     echo Source root reset to ''${sourceRoot}
   '';
 
+  preConfigure = ''
+    sed -i -e "s|/bin/sh|${stdenv.shell}|" configure
+  '';
+
   configureFlags = "--disable-debug";
 
   enableParallelBuilding = true;