change expat URL in README.md

Apparently we have readytalk.github.io set up to 301 redirect all URLs
to oss.readytalk.com, which confuses curl.  Rather than figure out how
to make curl do the right thing, I've just changed the URL to point to
oss.readytalk.com.
update android.pro for compatibility with recent upstream code
fix Android classpath build for iOS

This mainly involved reworking the makefile to avoid conflating
Darwin/ARM builds with iOS, since we may also want to build for the
iOS Simulator, which is i386.

Note that I was only able to test this on the Simulator, since I don't
have a real iOS device to test with.  Sorry if I broke something; if
so, please fix it :)