Merge bitcoin#579: Use __GNUC_PREREQ for detecting __builtin_expect

c663397 Use __GNUC_PREREQ for detecting __builtin_expect (Tim Ruffing)

Pull request description:

Tree-SHA512: 659a721da835eb15966a2a5386d6ae4c3defbb6ad473905f14161a3cf36d1556d44d887602019c89ebeffdfd82ed469ff27914dc2aaee6648d1e0c8a22151c27

configure.ac

@@ -153,12 +153,6 @@ AC_ARG_WITH([asm], [AS_HELP_STRING([--with-asm=x86_64|arm|no|auto]
 
 AC_CHECK_TYPES([__int128])
 
-AC_MSG_CHECKING([for __builtin_expect])
-AC_COMPILE_IFELSE([AC_LANG_SOURCE([[void myfunc() {__builtin_expect(0,0);}]])],
-    [ AC_MSG_RESULT([yes]);AC_DEFINE(HAVE_BUILTIN_EXPECT,1,[Define this symbol if __builtin_expect is available]) ],
-    [ AC_MSG_RESULT([no])
-    ])
-
 if test x"$enable_coverage" = x"yes"; then
     AC_DEFINE(COVERAGE, 1, [Define this symbol to compile out all VERIFY code])
     CFLAGS="$CFLAGS -O0 --coverage"

src/util.h

@@ -36,7 +36,7 @@ static SECP256K1_INLINE void secp256k1_callback_call(const secp256k1_callback *
 } while(0)
 #endif
 
-#ifdef HAVE_BUILTIN_EXPECT
+#if SECP256K1_GNUC_PREREQ(3, 0)
 #define EXPECT(x,c) __builtin_expect((x),(c))
 #else
 #define EXPECT(x,c) (x)