Soxr

* SIMD enhancements: SSE, AVX, Neon.
* Improve support for clang, ARM, and cross-compilation.
* Provide env. var. override of runtime parameters.
* Build fix re cmake variables AVCODEC_INCLUDE_DIRS & AVUTIL_INCLUDE_DIRS.
* Build options WITH_SINGLE_PRECISION, WITH_DOUBLE_PRECISION & WITH_SIMD have
been removed; replacement options are detailed in INSTALL, `Resampling
engines'.

* Fix conversion failure when I/O types differ but I/O rates don't.
* Fix defines for interpolation order selection.
* Fix ineffectual SOXR_MINIMUM_PHASE and SOXR_INTERMEDIATE_PHASE in
soxr_quality_spec recipe.
* Fix soxr_delay() returning a negative number after end-of-input has been
indicated.
* Fix crash when using soxr_process() after calling soxr_clear().
* Be more POSIX compliant w.r.t. errno in the examples; fixes erroneous
reporting of errors on FreeBSD.
* Quality improvement for variable-rate.
* Various fixes/improvements to build/tests/documentation.

* Minor fixes/improvements to build/tests.
* Fix crash (e.g. with k3b) when null error pointer passed to src_create (lsr
bindings only).
* Fix broken resampling in many cases with SIMD and anti_aliasing_pc < 100.
* For clarity, renamed and slightly changed usage of three parameters in
soxr_quality_spec_t (ABI compatible, API incompatible). An application not
setting these parameters directly need make no change; otherwise, changes
should be made per the following example (as shown, compatibility with both
old/new APIs is maintained). See also the comments on these parameters in
soxr.h. N.B. ABI compatibility with the 0.1.0 API may be removed in a
future release.
if !defined SOXR_VERSION /* Deprecated, 0.1.0 API */
q_spec.phase = minimum_phase? 0 : 50;
q_spec.bw_pc = cutoff * 100;
q_spec.anti_aliasing_pc = anti_aliasing * 100;
else /* 0.1.1 API */ Explanation:
q_spec.phase_response = minimum_phase? 0 : 50; Renamed.
q_spec.passband_end = cutoff; Renamed, no longer %.
q_spec.stopband_begin = 2 - anti_aliasing; Renamed, no longer %, no
endif longer mirrored in Fs.

* First public release.