>   # Set optimization level
>         if [[ ${ARCH} == hppa ]]; then
>                 mozconfig_annotate "more than -O0 causes a segfault on
> hppa" --enable-optimize=-O0
>         elif [[ ${ARCH} == x86 ]]; then
>                 mozconfig_annotate "less then -O2 causes a segfault on
> x86" --enable-optimize=-O2
>         elif use custom-optimization || [[ ${ARCH} =~ (alpha|ia64) ]]; then
>                 # Set optimization level based on CFLAGS
>                 if is-flag -O0; then
>                         mozconfig_annotate "from CFLAGS"
> --enable-optimize=-O0
>                 elif [[ ${ARCH} == ppc ]] && has_version
> '>=sys-libs/glibc-2.8'; then
>                         mozconfig_annotate "more than -O1 segfaults on
> ppc with glibc-2.8" --enable-optimize=-O1
>                 elif is-flag -O3; then
>                         mozconfig_annotate "from CFLAGS"
> --enable-optimize=-O3
>                 elif is-flag -O1; then
>                         mozconfig_annotate "from CFLAGS"
> --enable-optimize=-O1
>                 elif is-flag -Os; then
>                         mozconfig_annotate "from CFLAGS"
> --enable-optimize=-Os
>                 else
>                         mozconfig_annotate "Gentoo's default
> optimization" --enable-optimize=-O2
>                 fi
>         else
>                 # Enable Mozilla's default
>                 mozconfig_annotate "mozilla default" --enable-optimize
>         fi
> """
>

And just after that, it appears that if custom-cflags is not set, then it
strips all the -O* flags (if i'm reading the code correctly), which would
explain why there's no -O set for the compiler.....

        # Strip optimization so it does not end up in compile string
        filter-flags '-O*'

        # Strip over-aggressive CFLAGS
        use custom-cflags || strip-flags