Newer versions of emscipten, starting all the way back in 1.39.13, can
automatically locate the `.emscripten` config file that emsdk creates so
there is no need for the explicit EM_CONFIG environment variable. Its
redundant and adds unnessary noisce/complexity.
Really, adding emcc to the PATH is all the is needed these days.
One nice thing about this change is that it allows folks to run
whichever emcc they want to and have it just work, even if they have
configured emsdk. Without this change, if I activate emsdk and I run
`some/other/emcc` then emsdk's `EM_CONFIG` will still be present and
override the configuration embedded in `some/other/emcc`.
e.g. in the same shell, with emsdk activated, I can run both these
commands and have them both just work as expected.
```
$ emcc --version
$ /path/to/my/emcc --version
```
