[js] does not compile with emscripten >= 1.38

[uhoreg]

as 1.38 defaults to wasm, but wasm is apparently not compatible with our pre and post script somehow
(see also Kagami/ffmpeg.js#44 (comment) )

so we need to add -s WASM=0 to the EMCCFLAGS

[uhoreg]

even after adding -s WASM=0, there are still issues with emscripten 1.38

• Module['Runtime'] is no longer defined. stackSave and stackRestore seem to be in Module directly now, rather than in Module['Runtime'
• Module['allocate'] is no longer defined (it seems to be a global now?)
• even after fixing these issues, the unit tests fail, saying abort("Assertion failed: undefined"). ☹️

[dbkr]

There's now a pending PR in olm for this.