Speex 1.2.1 release

- Check for _WIN32 instead of WIN32 in preprocessor checks
- wav_io: check for EOF when seeking in wav (fixes hang discovered by fuzzing, see https://github.com/xiph/speex/issues/9)
- CI: add gitlab CI integration
- fixed-point: make left shift macros use unsigned to avoid undefined behaviour
- math_approx: use unsigned int for LCG pseudorandom generator (avoids integer overflow)
- oss-fuzz: add integration and fuzzing target
- speexenc: guard against invalid channel numbers (see https://github.com/xiph/speex/issues/13)
- speexdec: make left shift macros use unsigned to avoid undefined behaviour
- autotools: do not use deprecated macros
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