libunwind is included in libSystem; disable build, copy headers only

pkgs/os-specific/darwin/apple-source-releases/libunwind/default.nix

@@ -1,29 +1,11 @@
 { stdenv, appleDerivation, dyld, osx_private_sdk }:
 
 appleDerivation {
-  phases = [ "unpackPhase" "buildPhase" "installPhase" ];
+  phases = [ "unpackPhase" "installPhase" ];
-
-  buildInputs = [ dyld ];
-
-  NIX_CFLAGS_COMPILE = "-I${osx_private_sdk}/PrivateSDK10.9.sparse.sdk/usr/include";
-
-  buildPhase = ''
-    pushd src
-    c++ -I../include -c libuwind.cxx -o libuwind.o
-    cc  -I../include -c Registers.s -o Registers.o
-    cc  -I../include -c unw_getcontext.s -o unw_getcontext.o
-    cc  -I../include -c UnwindLevel1.c -o UnwindLevel1.o
-    cc  -I../include -c UnwindLevel1-gcc-ext.c -o UnwindLevel1-gcc-ext.o
-    cc  -I../include -c Unwind-sjlj.c -o Unwind-sjlj.o
-    ld -arch x86_64 -dylib libuwind.o Registers.o unw_getcontext.o UnwindLevel1.o UnwindLevel1-gcc-ext.o Unwind-sjlj.o \
-      -lc++ -lc -install_name $out/lib/libunwind.dylib -o libunwind.dylib
-    popd
-  '';
 
   installPhase = ''
     mkdir -p $out/lib
     cp -R include $out/include
-    install -m 0755 src/libunwind.dylib $out/lib
   '';
 
   meta = with stdenv.lib; {