linux-user: Fix compilation with clang 3.4

Clang version 3.4.2 does not know the -Wpragmas option yet and bails
out with an error when we try to disable it in linux-user/qemu.h.
Fortunately, clang has a __has_warning() macro which allows us to add
an explicit check for the option that we want to ignore. With that we
can check for the availability of "-Waddress-of-packed-member" properly
and do not need the "-Wpragmas" at all here.

Fixes: 850d5e330a9c68cc998cecc02caf8a3c8d1ee8a3
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Thomas Huth <thuth@redhat.com>

include/qemu/compiler.h

@@ -113,6 +113,10 @@
 #define GCC_FMT_ATTR(n, m)
 #endif
 
+#ifndef __has_warning
+#define __has_warning(x) 0 /* compatibility with non-clang compilers */
+#endif
+
 #ifndef __has_feature
 #define __has_feature(x) 0 /* compatibility with non-clang compilers */
 #endif

linux-user/qemu.h

@@ -474,17 +474,13 @@ static inline int access_ok(int type, abi_ulong addr, abi_ulong size)
  *   functions than host-endian unaligned load/store plus tswapN.
  * - The pragmas are necessary only to silence a clang false-positive
  *   warning: see https://bugs.llvm.org/show_bug.cgi?id=39113 .
- * - We have to disable -Wpragmas warnings to avoid a complaint about
- *   an unknown warning type from older compilers that don't know about
- *   -Waddress-of-packed-member.
  * - gcc has bugs in its _Pragma() support in some versions, eg
  *   https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83256 -- so we only
  *   include the warning-suppression pragmas for clang
  */
-#ifdef __clang__
+#if defined(__clang__) && __has_warning("-Waddress-of-packed-member")
 #define PRAGMA_DISABLE_PACKED_WARNING                                   \
     _Pragma("GCC diagnostic push");                                     \
-    _Pragma("GCC diagnostic ignored \"-Wpragmas\"");                    \
     _Pragma("GCC diagnostic ignored \"-Waddress-of-packed-member\"")
 
 #define PRAGMA_REENABLE_PACKED_WARNING          \