README for xalloc: a debug helper and alternative to omalloc

- substitute in <topdir>/configure.ac and <topdir>/Makefile.am
   omalloc by xalloc
- in <topdir> run ./autogen.sh
- run <topdir>/configure .....   in <builddir>
- in <builddir> and <topdir>: save omalloc to omalloc.org, ln -s xalloc omalloc
- run make in <builddir>
