about "cpu.h: Fix compiler detection" patch

After this patch, all FLAC__SSEN_SUPPORTED variables are
undefined for GCC, so intrinsic versions of functions are
not compiled into libFLAC.

Previously, the code was:

#if defined __INTEL_COMPILER
   // definitions for ICC
#elif defined _MSC_VER
   // definitions for MSVC
#elif defined __GNUC__ || defined __clang__
   #if defined __clang__ && __has_attribute(__target__)
     // definitions for newer clang
   #elif !defined __clang__ && (__GNUC__ > 4 || (__GNUC__ == 4 &&  
__GNUC_MINOR__ >= 9))
     // definitions for newer GCC
   #else
     // definitions for older GCC and clang
   #endif
#endif



Now, it's basically:



#if defined __INTEL_COMPILER
   // definitions for ICC
#elif defined _MSC_VER
   // definitions for MSVC
#elif defined __GNUC__ && defined __clang__
   // definitions for clang
#endif



By the way, what's the problem with GCC 4.6?
According to <https://gcc.gnu.org/onlinedocs/cpp/If.html>:
"... and logical operations (&& and ||). The latter two obey the usual  
short-circuiting rules of standard C."

So I thought that the directive

    #if defined __clang__ && __has_attribute(__target__)

is ok for GCC because "defined __clang__" is false and
preprocessor shouldn't try to parse "__has_attribute(...)" part.
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