6648 illumos build should be explicit about C standards

*** 182,192 ****
  
  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_AS_XARCH_32 = $(i386_AS_XARCH)
  DBOOT_AS_XARCH_64 = $(amd64_AS_XARCH)
  DBOOT_ASFLAGS = $(DBOOT_AS_XARCH_$(CLASS)) -P -D_ASM
  
--- 182,193 ----
  
  DBOOT_OBJS_DIR= dboot/$(OBJS_DIR)
  
  DBOOT_XARCH_32 = $(i386_XARCH)
  DBOOT_XARCH_64 = $(amd64_XARCH)
! 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