Rename configure.in to configure.ac to make more recent auto*s happy,
and also add the automake option "subdir-objects".  As advisde by
Tromey and Mjw.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13521 a5019735-40e9-0310-863c-91ae7b9d1cf9