Allow overriding compile parameters in pywasmcross

[joemarshall]

If you are building a package using pyodide build, as default, calls to cc etc. for compiling or linking are routed via pywasmcross.py, which adds on the build options from pyodide.

If you have compile options that differ from the build options of pyodide, then pywasmcross kills them. Most obviously if you are building a package as debug against a non-debug pyodide, it adds -g0 to the command line and your debug info disappears.

I can work around this for now by changing my build to use emcc directly instead of calling cc / c++ but it would be nice if pywasmcross didn't override debug flags that were already set on the command line.

[ryanking13]

Thanks for opening the issue @joemarshall. I think it makes sense. I'm planning various improvements to pyodide-build (#4557), part of which involves creating a separate config file that pyodide-build uses, which could contain compilation options.