configure: silence preprocessor noise from dependency generation

Currently, errors are thrown for various macros while building that are completely bogus.
They occur during the dependency (.d) generation phase, and have no bearing on the compiled output,
since only the stdout is piped into the sed command to generate the .d files.
They basically occur as the relevant -I paths are not (and cannot be passed) during
the dependancy generation phase.
As such, this patch silences them.
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

configure

@@ -2980,7 +2980,7 @@ 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) $< | sed -e "/^\#.*/d" -e "s,^[[:space:]]*$(*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)\\.o,$(@D)/$(*F).o," > $(@:.o=.d)'
 DEPFLAGS='-MM'
 
 # find source path