A typical incantation as observed in OpenEXR-1.0.5:
source='ImfAttribute.cpp' \
object='libIlmImf_la-ImfAttribute.lo' libtool=yes \
depfile='.deps/libIlmImf_la-ImfAttribute.Plo' \
tmpdepfile='.deps/libIlmImf_la-ImfAttribute.TPlo' \
depmode=sgi /bin/sh \
/apps/src/OpenEXR-1.0.5-INST-irix/depcomp \
        /bin/sh ../libtool --mode=compile CC ...

Now, depcomp adds
`-Wp,-MDupdate,.deps/libIlmImf_la-ImfAttribute.TPlo' to
the end of the command line before handing it to
libtool.  libtool, then, mistakes that option for a
source file.  This gets important when compiler_c_o==no
(compiler can't use -c and -o at the same time), and
the object file has a different basename than the
source ($CC -o newname.o -c oldname.c).

Relevant lines in libtool:

    # Calculate the filename of the output object if
compiler does
    # not support -o with -c
    if test "$compiler_c_o" = no; then
      output_obj=`$echo "X$srcfile" | $Xsed -e
's%^.*/%%' -e 's%\.[^.]*$%%'`.${o
bjext}
      lockfile="$output_obj.lock"
      removelist="$removelist $output_obj $lockfile"
      trap "$run $rm $removelist; exit 1" 1 2 15
    else