When trying to build the binary dct-test under MSYS2/Win64, the
makefile rule does not have the SUBDIR in the target for its
object file.

Consequently, modifications to various include files (e.g. C ones)
do not trigger a recompilation.

When tracing the dependency generating, the dependency generation
has this strange content (linebreak inserted):
sed -e "/^#.*/d" -e "s,^[[:space:]]*dct\\.o,libavcodec/dct.o," \
 > libavcodec/dct-test.d

For some reason, the $(*F) has weird content. It looks simpler to
use $(@F) instead of $(*F)\\.o, although this was tested on one
single version of make.
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>