Update Ubuntu instructions

[mcandre]

The WASM installation instructions recommend using a prebuilt "emscripten" binary package such as from Ubuntu [18.10 Cosmic]. However, this package appears to be broken out of the box:

$ emcc -s WASM=1 -o bin/hello.html hello.c
WARNING  root: LLVM version appears incorrect (seeing "(tags/RELEASE_700/final)", expected "3.4")
INFO     root: (Emscripten: Running sanity checks)
WARNING  root: java does not seem to exist, required for closure compiler. -O2 and above will fail. You need to define JAVA in ~/.emscripten
WARNING  root: Assigning a non-existent settings attribute "WASM"
WARNING  root:  - did you mean one of ASM_JS?
WARNING  root:  - perhaps a typo in emcc's  -s X=Y  notation?
WARNING  root:  - (see src/settings.js for valid values)
hello.c:34:5: warning: return type of 'main' is not 'int' [-Wmain-return-type]
    void main() {
    ^
hello.c:34:5: note: change return type to 'int'
    void main() {
    ^~~~
    int
1 warning generated.
/usr/bin/lli: error: error creating EE: No available targets are compatible with this triple.
FAIL: Running the generated program failed!

Is this package out of date? Could we push the Ubuntu community to update this package? And also temporarily recommend source-based installation while this gets fixed?

[binji]

Yes, it seems this package is too old. I'm not sure what to recommend instead, but @kripken @dschuff @jgravelle-google probably do.

[jgravelle-google]

EMSDK is generally the way we recommend: https://github.com/emscripten-core/emsdk