Check for bswap* builtins before using

Canonical builtin checks for clang are to use
__has_builtin. Much less fragile than version checks.

https://code.google.com/p/webm/issues/detail?id=1082

Change-Id: I8151fb75899acdf1a935c23aad9441da99a9abcd

vpx_util/endian_inl.h

diff --git a/vpx_util/endian_inl.h b/vpx_util/endian_inl.h
index 6b177f1..37bdce1 100644
--- a/vpx_util/endian_inl.h
+++ b/vpx_util/endian_inl.h
@@ -25,14 +25,10 @@
 # define LOCAL_GCC_PREREQ(maj, min) 0
 #endif
 
-#ifdef __clang__
-# define LOCAL_CLANG_VERSION ((__clang_major__ << 8) | __clang_minor__)
-# define LOCAL_CLANG_PREREQ(maj, min) \
-    (LOCAL_CLANG_VERSION >= (((maj) << 8) | (min)))
-#else
-# define LOCAL_CLANG_VERSION 0
-# define LOCAL_CLANG_PREREQ(maj, min) 0
-#endif  // __clang__
+// handle clang compatibility
+#ifndef __has_builtin
+# define __has_builtin(x) 0
+#endif
 
 // some endian fix (e.g.: mips-gcc doesn't define __BIG_ENDIAN__)
 #if !defined(WORDS_BIGENDIAN) && \
@@ -53,16 +49,18 @@
 #define HToBE32(X) BSwap32(X)
 #endif
 
-// clang-3.3 and gcc-4.3 have builtin functions for swap32/swap64
-#if LOCAL_GCC_PREREQ(4, 3) || LOCAL_CLANG_PREREQ(3, 3)
-#define HAVE_BUILTIN_BSWAP32
-#define HAVE_BUILTIN_BSWAP64
-#endif
-// clang-3.3 and gcc-4.8 have a builtin function for swap16
-#if LOCAL_GCC_PREREQ(4, 8) || LOCAL_CLANG_PREREQ(3, 3)
+#if LOCAL_GCC_PREREQ(4, 8) || __has_builtin(__builtin_bswap16)
 #define HAVE_BUILTIN_BSWAP16
 #endif
 
+#if LOCAL_GCC_PREREQ(4, 3) || __has_builtin(__builtin_bswap32)
+#define HAVE_BUILTIN_BSWAP32
+#endif
+
+#if LOCAL_GCC_PREREQ(4, 3) || __has_builtin(__builtin_bswap64)
+#define HAVE_BUILTIN_BSWAP64
+#endif
+
 #if HAVE_MIPS32 && defined(__mips__) && !defined(__mips64) && \
     defined(__mips_isa_rev) && (__mips_isa_rev >= 2) && (__mips_isa_rev < 6)
 #define VPX_USE_MIPS32_R2