- Disable external speex, since it was causing audio hiccups and
displaying warning messages in loop.
- Only install the pjsua binary. The build system always builds it as
a static binary anyway, even when using `--enable-shared`. Perhaps there
should be a separated pjsip package for the libraries.
- Introduce the `-NDEBUG` flag, which the README recommends for release
- Introduce the `-DPJ_TIMER_DEBUG=0` flag. This is a workaround for
building version 2.10. Linking sometimes fails missing some `*_dbg`
symbols. Sometimes it builds sucessfully even without the flag. Maybe
there is a race condition in the build system.