build-sys: Make speex library optional

build-sys: Make speex library optional

make speex library dependency optional, this affects the resampler
and the echo canceller module

this patch supersedes an earlier patch proposal and addresses the following
comments:
* fix order of pa_echo_canceller_method_t enum and ec_table (Frederic)
* the default resampler is speex if available as before, otherwise ffmpeg (Arun)
* does not touch the Adrian EC implementation (see separate patch) (Arun)