pc-bios/s390-ccw/netboot.mak: Ignore Clang's warnings about GNU extensions

When compiling the s390-ccw bios with Clang (v14.0), there is currently
an unuseful warning like this:

  CC      pc-bios/s390-ccw/ipv6.o
 ../../roms/SLOF/lib/libnet/ipv6.c:447:18: warning: variable length array
  folded to constant array as an extension [-Wgnu-folding-constant]
                unsigned short raw[ip6size];
                               ^

SLOF is currently GCC-only and cannot be compiled with Clang yet, so
it is expected that such extensions sneak in there - and as long as
we don't want to compile the code with a compiler that is neither GCC
or Clang, it is also not necessary to avoid such extensions.

Thus these GNU-extension related warnings are completely useless in
the s390-ccw bios, especially in the code that is coming from SLOF,
so we should simply disable the related warnings here now.

Message-Id: <20220704111903.62400-13-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

pc-bios/s390-ccw/netboot.mak

@@ -16,9 +16,12 @@ s390-netboot.elf: $(NETOBJS) libnet.a libc.a
 s390-netboot.img: s390-netboot.elf
 	$(call quiet-command,$(STRIP) --strip-unneeded $< -o $@,"STRIP","$(TARGET_DIR)$@")
 
+# SLOF is GCC-only, so ignore warnings about GNU extensions with Clang here
+NO_GNU_WARN := $(call cc-option,-Werror $(QEMU_CFLAGS),-Wno-gnu)
+
 # libc files:
 
-LIBC_CFLAGS = $(QEMU_CFLAGS) $(CFLAGS) $(LIBC_INC) $(LIBNET_INC) \
+LIBC_CFLAGS = $(QEMU_CFLAGS) $(CFLAGS) $(NO_GNU_WARN) $(LIBC_INC) $(LIBNET_INC) \
 	      -MMD -MP -MT $@ -MF $(@:%.o=%.d)
 
 CTYPE_OBJS = isdigit.o isxdigit.o toupper.o
@@ -52,7 +55,7 @@ libc.a: $(LIBCOBJS)
 
 LIBNETOBJS := args.o dhcp.o dns.o icmpv6.o ipv6.o tcp.o udp.o bootp.o \
 	      dhcpv6.o ethernet.o ipv4.o ndp.o tftp.o pxelinux.o
-LIBNETCFLAGS = $(QEMU_CFLAGS) $(CFLAGS) $(LIBC_INC) $(LIBNET_INC) \
+LIBNETCFLAGS = $(QEMU_CFLAGS) $(CFLAGS) $(NO_GNU_WARN) $(LIBC_INC) $(LIBNET_INC) \
 	       -DDHCPARCH=0x1F -MMD -MP -MT $@ -MF $(@:%.o=%.d)
 
 %.o : $(SLOF_DIR)/lib/libnet/%.c