build: Avoid libstdc++ dep for module-echo-cancel if possible

build: Avoid libstdc++ dep for module-echo-cancel if possible

This moves out the webrtc bits into a small helper library to shield the
rest of module-echo-cancel from being linked with a C++ linker. This is
required because automake will _always_ link module-echo-cancel in C++
mode if any of its deps (even conditional ones) are in C++.