build system: fix dependency generation

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>

configure

@@ -2984,7 +2984,7 @@ target_path='$(CURDIR)'
 
 # since the object filename is not given with the -MM flag, the compiler
 # is only able to print the basename, and we must add the path ourselves
-DEPCMD='$(DEP$(1)) $(DEP$(1)FLAGS) $($(1)DEP_FLAGS) $< 2>/dev/null | sed -e "/^\#.*/d" -e "s,^[[:space:]]*$(*F)\\.o,$(@D)/$(*F).o," > $(@:.o=.d)'
+DEPCMD='$(DEP$(1)) $(DEP$(1)FLAGS) $($(1)DEP_FLAGS) $< 2>/dev/null | sed -e "/^\#.*/d" -e "s,^[[:space:]]*$(@F),$(@D)/$(@F)," > $(@:.o=.d)'
 DEPFLAGS='-MM'
 
 # find source path