Fix dependency generation for older gcc versions

With gcc 3.3.3 at least, compilation fails with

Generating include/autoconf.mk
gcc: compilation of header file requested
make: *** [include/autoconf.mk] Error 1

since commit 16fe77752e.

Signed-off-by: Vlad Lungu <vlad@comsys.ro>

Makefile

@@ -432,7 +432,7 @@ $(obj)include/autoconf.mk: $(obj)include/config.h $(VERSION_FILE)
 	@$(XECHO) Generating include/autoconf.mk ; \
 	set -e ; \
 	: Generate the dependancies ; \
-	$(CC) -M $(HOST_CFLAGS) $(CPPFLAGS) -MQ $@ include/common.h > $@.dep ; \
+	$(CC) -x c -M $(HOST_CFLAGS) $(CPPFLAGS) -MQ $@ include/common.h > $@.dep ; \
 	: Extract the config macros ; \
 	$(CPP) $(CFLAGS) -dM include/common.h | sed -n -f tools/scripts/define2mk.sed > $@