-march=native and build failures

Hi,
  I've traced down at least some build failures to our use of -march=native.
What apparently happens is that in some ocasions gcc emits some mmx
instructions which fail with a SIGILL.

We have a few choices:
1- leave the builds broken for now and try rework the code
2- add an --enable-extra-compiler-optimizations configure switch to
optionally enable -march=native instead of unconditionally.
3- work around this on the broken nodes and on those only by using
build trickery.

Thoughts?
I'd favor option 2'; we can then leave a specifically-broken build job
so that when some change we make helps fix the build, we will know.

-- 
    /kinkie