Refactor WASMStandaloneDevice Build: Drop distutils, add native emcc prefs

[PalashChitnavis]

Fixes #19

This patch makes the WASM backend talk straight to emcc instead of first trying to use GCC or MSVC. It adds two new preference lists where you can set your own emcc compile- and link-flags. The build code now writes the right -I, -L, and -l flags itself, so no more post-hacking or flag cleanup is needed. Result: you only need Python and the Emscripten SDK—no Visual Studio or system compiler required.

[PalashChitnavis]

Hi @mstimberg, is there anything else you’d like me to change in this PR?

[mstimberg]

Hi @PalashChitnavis Sorry, I wasn't much available the last days. Your changes look good, but here are a few suggestions for things to change/add:

• the README file should state the preferences that are used and what they are used for (things like header files and library dirs from the standard Brian2 preferences, and the emsdk-specific ones from Brian2Wasm)
• the things about contributions and license are useful boilerplate but: 1) we don't use conventional commit messages, and 2) Brian2's license is not BSD-style but rather GPL-style (also, this repo does not actually have a LICENSE file 😬 )
• the documentation for the CLI is currently a docstring, but it would be good to include it in the README file as well.

[mstimberg]

LGTM, we can do the README changes later