osx: don't try to build universal binaries

The use of multiple arch is not safe with autotools. I believe that
libevent compute some arch-dependent stuff at configure-time. Just say
in README.md that the package will only work for the same version of
OSX and the same architecture.

For the future:

  - real universal binaries can be built by configuring and building
    each arch into a separate directory, then merge the result with
    `lipo`.

  - building a package for an older versions of Max OS X can be done
    by using `-mmacosx-version-min=10.6` and `-isysroot
    /Developer/SDKs/MacOSX10.5.sdk` on both `CFLAGS` and `LDFLAGS`.