Revert "Fix icu4c expression for latest gcc (fixes #2341)"

This reverts commit d393c6c538.
The commit removed C++11 compatibility on darwin by overriding the
--std=c++0x flag in CXXFLAGS. Which lead to a failing build of mapnik,
which depends on the move constructors being available in the icu-lib.
Since it builds fine without the headerpad_max_install_names flag, we
simply undo the change that introduced this flag.

pkgs/development/libraries/icu/default.nix

@@ -16,9 +16,6 @@ stdenv.mkDerivation ({
   outputs = [ "out" "dev" ];
   outputBin = "dev";
 
-  makeFlags = stdenv.lib.optionalString stdenv.isDarwin
-    "CXXFLAGS=-headerpad_max_install_names";
-
   # FIXME: This fixes dylib references in the dylibs themselves, but
   # not in the programs in $out/bin.
   buildInputs = stdenv.lib.optional stdenv.isDarwin fixDarwinDylibNames;