[libvorbis patch] -O20 and -O4 don't exist (emscripten JS cross-compile fix)

The configure.ac script for libvorbis specifies optimization levels of -O20 and -O4 in various situations; however both gcc and clang only support up to -O3 and silently clamp larger values.

More annoyingly, the emscripten JavaScript cross-compiler outright rejects such options, breaking the build with unmodified SVN trunk.

May as well just use -O3. :)