6648 illumos build should be explicit about C standards

@@ -182,11 +182,12 @@
 
 DBOOT_OBJS_DIR= dboot/$(OBJS_DIR)
 
 DBOOT_XARCH_32 = $(i386_XARCH)
 DBOOT_XARCH_64 = $(amd64_XARCH)
-DBOOT_CFLAGS = $(DBOOT_XARCH_$(CLASS)) $(CERRWARN) $(CCNOAUTOINLINE) -O
+DBOOT_CFLAGS = $(DBOOT_XARCH_$(CLASS)) $(C99MODE) $(CERRWARN) \
+    $(CCNOAUTOINLINE) -O
 
 DBOOT_AS_XARCH_32 = $(i386_AS_XARCH)
 DBOOT_AS_XARCH_64 = $(amd64_AS_XARCH)
 DBOOT_ASFLAGS = $(DBOOT_AS_XARCH_$(CLASS)) -P -D_ASM