Changes

configure: handle emscripten builds and disable unsupported features

Detect emscripten compiler and adjust default configure options
accordingly. When building with emcc, disable ortp and DAHDI
support by default, as these components are not applicable in
Emscripten environments.

Also ignore generated .wasm artifacts in .gitignore.

No functional change for native builds.

Change-Id: I879e698f2d514cf6e2f7bb33b4d6148f79aea6f6