When both a .o and .lo file was used in a Makefile, we used to make

When both a .o and .lo file was used in a Makefile, we used to make
the .o depend on the .lo.  Unfortunately, this creates a race
condition for parallel make since libtool is not atomic (it creates
a .o and then renames it when building PIC objects for shared libs).

We always link with libtool so the only reason to prefer the .o
over the .lo file is to avoid mixing .o and .lo in the dependencies.
That's not a good enough reason so change mkdep.pl to warn when
both a .o and .lo are referenced in a Makefile and do nothing else.

Bug #842