Build error on macos x

Hi
The following error after running `jpm build`

```
git clone https://github.com/janet-lang/jaylib
cd jaylib/
git submodule update --init --recursive
jpm build
compiling build/src___main.o...
cc1: error: argument to '-O' should be a non-negative integer, 'g', 's' or 'fast'
error: command exited with status 1
in shell [/Users/ben/builds/bin/jpm] (tailcall) on line 264, column 5
in do-rule [/Users/ben/builds/bin/jpm] on line 100, column 24
in do-rule [/Users/ben/builds/bin/jpm] on line 98, column 44
in do-rule [/Users/ben/builds/bin/jpm] (tailcall) on line 98, column 44
```

My system:

ProductName: Mac OS X
ProductVersion: 10.15.4
BuildVersion: 19E287

cc --version
cc (Homebrew GCC 9.3.0_1) 9.3.0

Reproduce the failure in the jaylib repository by following the reported macOS setup and running `jpm build` with Homebrew GCC 9.3.0. Start from the build command that produces the invalid `-O` argument, then verify that the project builds successfully on the reported environment.

Written by the indexing model from the issue text.