cpu.h: Fix compiler detection

GCC version 4.6 was throwing an error on `__has_attribute` which
seems to be Clang specific. Solution was to un-nest the CPP code
so that Clang is confirmed before the `__has_attribute` is hit.

src/libFLAC/include/private/cpu.h

@@ -88,7 +88,7 @@
     #define FLAC__AVX2_SUPPORTED 1
     #define FLAC__FMA_SUPPORTED 1
   #endif
-#elif defined __GNUC__ || defined __clang__
+#elif defined __GNUC__ && defined __clang__
   #if defined __clang__ && __has_attribute(__target__) /* clang */
     #define FLAC__SSE_TARGET(x) __attribute__ ((__target__ (x)))
     #if __has_builtin(__builtin_ia32_maxps)
@@ -106,7 +106,7 @@
     #if __has_builtin(__builtin_ia32_pabsd256)
       #define FLAC__AVX2_SUPPORTED 1
     #endif
-  #elif !defined __clang__ && (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 9)) /* GCC 4.9+ */
+#elif defined (__GNUC__) &&  (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 9)) /* GCC 4.9+ */
     #define FLAC__SSE_TARGET(x) __attribute__ ((__target__ (x)))
     #define FLAC__SSE_SUPPORTED 1
     #define FLAC__SSE2_SUPPORTED 1