Add configure test for __builtin_expect

Fixes #1695
Signed-off-by: Pavan Balaji <balaji@mcs.anl.gov>

confdb/aclocal_cc.m4

@@ -1651,3 +1651,21 @@ AC_DEFUN([PAC_C_MACRO_VA_ARGS],[
     fi
     rm -f pac_test.log
 ])dnl
+
+# Will AC_DEFINE([HAVE_BUILTIN_EXPECT]) if the compiler supports __builtin_expect.
+AC_DEFUN([PAC_C_BUILTIN_EXPECT],[
+AC_MSG_CHECKING([if C compiler supports __builtin_expect])
+
+AC_TRY_LINK(, [
+    return __builtin_expect(1, 1) ? 1 : 0
+], [
+    have_builtin_expect=yes
+    AC_MSG_RESULT([yes])
+], [
+    have_builtin_expect=no
+    AC_MSG_RESULT([no])
+])
+if test x$have_builtin_expect = xyes ; then
+    AC_DEFINE([HAVE_BUILTIN_EXPECT], [1], [Define to 1 if the compiler supports __builtin_expect.])
+fi
+])

configure.ac

@@ -2738,6 +2738,7 @@ AC_C_RESTRICT
 AC_C_INLINE
 
 PAC_C_GNU_ATTRIBUTE
+PAC_C_BUILTIN_EXPECT
 # We need to check for the endianess in order to implement the
 # "external32" representations.  This defines "WORDS_BIGENDIAN when
 # the system is bigendian.

src/include/mpiutil.h

@@ -338,8 +338,7 @@ int MPIR_Assert_fail_fmt(const char *cond, const char *file_name, int line_num,
  *
  * These macros are not namespaced because the namespacing is cumbersome.
  */
-/* safety guard for now, add a configure check in the future */
-#if ( defined(__GNUC__) && (__GNUC__ >= 3) ) || defined(__xlc__)
+#ifdef HAVE_BUILTIN_EXPECT
 #  define unlikely(x_) __builtin_expect(!!(x_),0)
 #  define likely(x_)   __builtin_expect(!!(x_),1)
 #else