Can't pass compiler flags to `nim build`
Hey, Nim newbie here. I've been scouring the docs and haven't found a solution for this, so if there is one then perhaps it can be documented more visibly. :)

I have a project which I primarily test by running natively, but my main goal is compiling to webassembly via Emscripten. I have the following nim.cfg snagged from the forum:

```
@if emscripten:
  cc = clang
  clang.exe = "emcc"
  clang.linkerexe = "emcc"
  clang.options.linker = ""
  cpu = "i386"
  passC = "-s WASM=1 -s 'BINARYEN_METHOD=\"native-wasm\"' -Iemscripten"
  passL = "-s WASM=1 -Lemscripten -s ALLOW_MEMORY_GROWTH=1"
@end
```

I also have a config.nims which does something similar.

Here's where my newbie flag flies high. :) I assume I want to put my project in a Nimble package, even if I don't publish it, so I can use Nim dependencies. If I want to build my code with those dependencies, I have to run `nimble build`. But as of now, I see no way to run something like `nimble build -d:emscripten` so the environment is correctly set up for Emscripten builds. I can use `nim c -d:emscripten ...` just fine, but then I won't be able to use Nim native dependencies. And maybe `nimble c ...` works, but again, I don't see that I can pass `-d:emscripten` for the times when I'm ready to build to wasm.

It seems my choice is either use Nimble, Nim dependencies, and pick one target, or use `nim c`, no Nim dependencies, and have the flexibility to switch targets. Is that accurate, or is there some hidden flag to pass command line parameters to `nimble build`? :)

Or should I not be using defines for this?

Thanks.