disas: Fix ATTRIBUTE_UNUSED define clash with ALSA headers

disas/bfd.h defines ATTRIBUTE_UNUSED, but unfortunately the
ALSA system headers also define this macro, which means that
you can get a compilation failure if building with ALSA and
any files happen to include the alsa headers before bfd.h
rather than the other way around.

This is unfortunate namespace pollution by the ALSA headers but
we can work around it. Add an #ifndef guard to bfd.h and remove
the unnecessary extra definition in disas/arm.c to fix this.

Reported-by: BALATON Zoltan <balaton@eik.bme.hu>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 1468937076-21503-1-git-send-email-peter.maydell@linaro.org

disas/arm.c

@@ -24,7 +24,6 @@
 
 #include "qemu/osdep.h"
 #include "disas/bfd.h"
-#define ATTRIBUTE_UNUSED __attribute__((unused))
 #define ISSPACE(x) ((x) == ' ' || (x) == '\t' || (x) == '\n')
 
 #define ARM_EXT_V1	 0

include/disas/bfd.h

@@ -477,7 +477,9 @@ int generic_symbol_at_address(bfd_vma, struct disassemble_info *);
   (INFO).disassembler_options = NULL, \
   (INFO).insn_info_valid = 0
 
+#ifndef ATTRIBUTE_UNUSED
 #define ATTRIBUTE_UNUSED __attribute__((unused))
+#endif
 
 /* from libbfd */