Add the 'subdir-objects' option in the automake makefile.

Add the 'subdir-objects' option in the automake makefile.

This rearranges the object files so that they each live alongside
their original source file, instead of all being in the same
directory. To my way of thinking this is a more or less neutral change
(perhaps marginally less tidy), but autotools is apparently beginning
to think it's the One True Way and 1.14 will give a warning if you
don't have it enabled.

[originally from svn r10142]