Use the libspeexdsp resampler

AFAICT it runs a little slower? but it's fixed point, and has much
better understood audio characteristics.

lib/speexdsp/TODO

@@ -0,0 +1,32 @@
+For 1.2:
+Major points:
+- Make documentation match the actual code (especially jitter buffer, AEC and preprocessor)
+- Get AGC to work in fixed-point even if not totally converted
+- Stabilise all APIs (need feedback)
+- Short-term estimate in jitter buffer
+- Control delay in new AEC API.
+- NaN checks?
+- Better error reporting
+- Make kiss-fft 32-bit safe
+
+Post 1.2:
+improve float<->int conversion
+split encoder and decoder?
+Merge TriMedia stuff
+packet dump
+Do VAD properly
+--enable-{aec,preprocessor,jitter,resampler}
+
+Optimisations
+- Add restrict in a few places?
+- use __builtin_expect() (likely()/unlikely())
+
+Allocator override (speex_lib_ctl?)
+Fixed-point:
+  - VBR
+  - Jitter buffer
+  - AGC
+Denoiser:
+  - Better noise adaptation
+AGC:
+  - Use median filtering instead of "non-linear mean"?