audio/fossmixer: fix illegal instruction error on some hosts

Previously, configure script was passing certain optimization options
by default; now it is protected by --enable-native switch which users
have to specify explicitly:

-C_FLAGS=$C_FLAGS" -O3 -ffast-math `/bin/sh ./build/opt.sh \"$CC\" -march=native`"
+C_FLAGS=$C_FLAGS" -O2"

Take one step further and replace -O2 with our default CFLAGS which are
essentially the same, but safer (https://blog.regehr.org/archives/1307).

We must, however, force -msse2 on i386 for the build to succeed there;
other architectures might or might not need similar treatment as well.

PR: 257520
Submitted by: maintainer (who is also upstream author)