Changes

configure: disable libsctp by default when building with emscripten

Detect the emscripten compiler and introduce a HAVE_EMSCRIPTEN
conditional.

When building with emscripten, disable libsctp by default, as SCTP is
not supported in that environment. Native builds keep libsctp enabled
by default.

Also ignore generated .wasm artifacts.

Change-Id: I023158cc39989d6fe98080fecb61410aa4e535ab