fix building w/gcc-5

--- a/libsandbox/Makefile.in
+++ b/libsandbox/Makefile.in
@@ -68,7 +68,7 @@ sb_nr.h: symbols.h $(SB_NR_FILE)
 
 TRACE_MAKE_HEADER = \
 	$(SB_AWK) $(GEN_TRACE_SCRIPT) -v MODE=gen | \
-		$(COMPILE) -E -include $(top_srcdir)/headers.h - $$f | \
+		$(COMPILE) -E -P -include $(top_srcdir)/headers.h - $$f | \
 		$(SB_AWK) $(GEN_TRACE_SCRIPT) -v syscall_prefix=$$t > $$header
 trace_syscalls.h: $(GEN_TRACE_SCRIPT) Makefile
 if SB_SCHIZO