libc++5: tweak expression so as to avoid rebuild on non-musl for now

pkgs/development/compilers/llvm/5/libc++/default.nix

@@ -9,9 +9,13 @@ stdenv.mkDerivation rec {
     unpackFile ${libcxxabi.src}
     export LIBCXXABI_INCLUDE_DIR="$PWD/$(ls -d libcxxabi-${version}*)/include"
   '';
-  patches = stdenv.lib.optionals stdenv.hostPlatform.isMusl [
+
+  # on next rebuild, this can be replaced with optionals; for now set to null to avoid
+  # patches = stdenv.lib.optionals stdenv.hostPlatform.isMusl [
+  patches = if stdenv.hostPlatform.isMusl then [
     ../../libcxx-0001-musl-hacks.patch
-  ];
+  ] else null;
+
   prePatch = ''
     substituteInPlace lib/CMakeLists.txt --replace "/usr/lib/libc++" "\''${LIBCXX_LIBCXXABI_LIB_PATH}/libc++"
   '';