compiler.h: explicit case for Clang printf attribute

Since commit efc6c070 ("configure: Add a test for the minimum compiler
version"), QEMU explicitely depends on GCC >= 4.8, we could thus drop
earlier version checks. Except clang advertizes itself as GCC 4.2.1.

Since clang doesn't support gnu_printf, make that case explicitely and
drop GCC version check.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20201210134752.780923-8-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/qemu/compiler.h

@@ -99,18 +99,18 @@
 #define QEMU_BUILD_BUG_ON_ZERO(x) (sizeof(QEMU_BUILD_BUG_ON_STRUCT(x)) - \
                                    sizeof(QEMU_BUILD_BUG_ON_STRUCT(x)))
 
-#if defined __GNUC__
-# if !QEMU_GNUC_PREREQ(4, 4)
-   /* gcc versions before 4.4.x don't support gnu_printf, so use printf. */
-#  define GCC_FMT_ATTR(n, m) __attribute__((format(printf, n, m)))
-# else
-   /* Use gnu_printf when supported (qemu uses standard format strings). */
-#  define GCC_FMT_ATTR(n, m) __attribute__((format(gnu_printf, n, m)))
-#  if defined(_WIN32)
-    /* Map __printf__ to __gnu_printf__ because we want standard format strings
-     * even when MinGW or GLib include files use __printf__. */
-#   define __printf__ __gnu_printf__
-#  endif
+#if defined(__clang__)
+/* clang doesn't support gnu_printf, so use printf. */
+# define GCC_FMT_ATTR(n, m) __attribute__((format(printf, n, m)))
+#elif defined(__GNUC__)
+/* Use gnu_printf (qemu uses standard format strings). */
+# define GCC_FMT_ATTR(n, m) __attribute__((format(gnu_printf, n, m)))
+# if defined(_WIN32)
+/*
+ * Map __printf__ to __gnu_printf__ because we want standard format strings even
+ * when MinGW or GLib include files use __printf__.
+ */
+#  define __printf__ __gnu_printf__
 # endif
 #else
 #define GCC_FMT_ATTR(n, m)