libavutil: Always use some GCC style attributes on clang

Clang normally disguises as GCC (defining __GNUC__), and thus get
all the normal GCC specific attributes.

Clang can also work as a drop-in replacement for MSVC, and in these
cases, it doesn't define __GNUC__, but defines _MSC_VER instead.

Even in these setups, it still supports the GCC style attributes,
thus use them, especially where there isn't any MSVC specific
version, or where the MSVC specific version doesn't work on clang
(for DECLARE_ASM_CONST).

Signed-off-by: Martin Storsjö <martin@martin.st>

libavutil/attributes.h

@@ -80,7 +80,7 @@
 #    define attribute_deprecated
 #endif
 
-#if defined(__GNUC__)
+#if defined(__GNUC__) || defined(__clang__)
 #    define av_unused __attribute__((unused))
 #else
 #    define av_unused
@@ -91,7 +91,7 @@
  * away.  This is useful for variables accessed only from inline
  * assembler without the compiler being aware.
  */
-#if AV_GCC_VERSION_AT_LEAST(3,1)
+#if AV_GCC_VERSION_AT_LEAST(3,1) || defined(__clang__)
 #    define av_used __attribute__((used))
 #else
 #    define av_used

libavutil/mem.h

@@ -48,7 +48,7 @@
     #define DECLARE_ASM_CONST(n,t,v)                    \
         AV_PRAGMA(DATA_ALIGN(v,n))                      \
         static const t __attribute__((aligned(n))) v
-#elif defined(__GNUC__)
+#elif defined(__GNUC__) || defined(__clang__)
     #define DECLARE_ALIGNED(n,t,v)      t __attribute__ ((aligned (n))) v
     #define DECLARE_ASM_CONST(n,t,v)    static const t av_used __attribute__ ((aligned (n))) v
 #elif defined(_MSC_VER)