speex: sync m4 with libopus m4
Namely:
- In addition to --with-speex=PREFIX, allow --with-speex=yes (to use the speex library from --speex-libraries=DIR and includes from --speex-includes=DIR, or from the default system directories) or --with-speex=no (to disable use of speex), to match opus.m4
- Fix -Wstrict-prototypes
- Fix misspelling