Can't build imgui on OSX with nim 1.4+
* **What is the current behavior?**

It fails while trying to build my imgui based nim project using a nim version greater than 1.06 (I think, 1.4+ for sure)

* **Please provide the steps to reproduce and if possible a minimal demo of the problem**

I've yet to try the C version (I did not compile cimgui) but the CPP build returns

```ruby
error: invalid argument '-std=gnu++14' not allowed with 'Objective-C'
```

Here is my compile command just in case some flags are missing/not supported:
```ruby
nim cpp --threads:on --outdir:dist/bin -r src/mtb/bin/test_imgui.nim
```
* **Please tell us about your environment**

  - Version: 1.4.2
  - OS: MacOS
  - Bindings: OpenGL & GLFW & ImGui 
  - Backend: CPP

* **Other information** (e.g. detailed explanation, stack traces, related issues, suggestions on how to fix, links for us to have context, eg. StackOverflow, docs.gl, documentation, etc)

Is there a way to use the [metal backend](https://github.com/floooh/sokol-samples/blob/master/metal/imgui-metal.cc) I'm not sure how this could be done using Nim?

