Passing a -g1 or -g2 flag at compile time doesn't do anything

Passing -g1 to clang at compile time gives a reduced amount of debug information. In emscripten, it gets filtered out and doesn't do anything.

[currently](https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-gline-tables-only) it looks like -g1 is just equivalent to `-gmlt` and `-gline-tables-only` (although I'm pretty sure this wasn't always the case, and I also don't think it's the case in gcc). So there is a workaround for this (also `-gline-tables-only` is what our docs recommend to users for this use case). But users might already have `-g1` in their build files, so we should probably fix this.