Enable by default the _sync version of atomic operations on OS X.

config/opal_config_asm.m4

@@ -2,7 +2,7 @@ dnl
 dnl Copyright (c) 2004-2005 The Trustees of Indiana University and Indiana
 dnl                         University Research and Technology
 dnl                         Corporation.  All rights reserved.
-dnl Copyright (c) 2004-2005 The University of Tennessee and The University
+dnl Copyright (c) 2004-2015 The University of Tennessee and The University
 dnl                         of Tennessee Research Foundation.  All rights
 dnl                         reserved.
 dnl Copyright (c) 2004-2006 High Performance Computing Center Stuttgart,
@@ -864,16 +864,26 @@ AC_DEFUN([OPAL_CONFIG_ASM],[
 
     AC_ARG_ENABLE([builtin-atomics],
       [AC_HELP_STRING([--enable-builtin-atomics],
-         [Enable use of __sync builtin atomics (default: disabled)])])
-
+         [Enable use of __sync builtin atomics (default: enabled)])]
+         [], [enable_builtin_atomics="yes"])
     AC_ARG_ENABLE([osx-builtin-atomics],
       [AC_HELP_STRING([--enable-osx-builtin-atomics],
-         [Enable use of OSX builtin atomics (default: disabled)])])
+         [Enable use of OSX builtin atomics (default: enabled)])],
+         [], [enable_osx_builtin_atomics="yes"])
 
-    if test "$enable_builtin_atomics" = "yes" ; then
-       OPAL_CHECK_SYNC_BUILTINS([opal_cv_asm_builtin="BUILTIN_SYNC"],
-         [AC_MSG_ERROR([__sync builtin atomics requested but not found.])])
-       AC_DEFINE([OPAL_C_GCC_INLINE_ASSEMBLY], [1],
+    opal_cv_asm_builtin="BUILTIN_NO"
+    if test "$enable_osx_builtin_atomics" = "yes" ; then
+       AC_MSG_CHECKING([for OSX atomic support])
+       AC_CHECK_HEADER([libkern/OSAtomic.h],
+                       [opal_cv_asm_builtin="BUILTIN_OSX"
+                        AC_MSG_RESULT([yes])],
+                       [AC_MSG_RESULT([no])])
+    fi
+    if test "$opal_cv_asm_builtin" = "BUILTIN_NO" -a "$enable_builtin_atomics" = "yes" ; then
+       AC_MSG_CHECKING([for builtin atomic support])
+       OPAL_CHECK_SYNC_BUILTINS([opal_cv_asm_builtin="BUILTIN_SYNC"
+                                 AC_MSG_RESULT([yes])],
+                                [AC_MSG_RESULT([no])])AC_DEFINE([OPAL_C_GCC_INLINE_ASSEMBLY], [1],
          [Whether C compiler supports GCC style inline assembly])
        OPAL_CHECK_SYNC_BUILTIN_CSWAP_INT128
     elif test "$enable_osx_builtin_atomics" = "yes" ; then