Update libunwind configuration check to look for unw_backtrace().

Update libunwind configuration check to look for unw_backtrace(), which
is a newer API not available in older versions of libunwind.
diff --git a/configure.ac b/configure.ac
index dc817e1..eb9ca45 100644
--- a/configure.ac
+++ b/configure.ac
@@ -702,7 +702,7 @@
 if test "x$backtrace_method" = "x" -a "x$enable_prof_libunwind" = "x1" ; then
   AC_CHECK_HEADERS([libunwind.h], , [enable_prof_libunwind="0"])
   if test "x$LUNWIND" = "x-lunwind" ; then
-    AC_CHECK_LIB([unwind], [backtrace], [LIBS="$LIBS $LUNWIND"],
+    AC_CHECK_LIB([unwind], [unw_backtrace], [LIBS="$LIBS $LUNWIND"],
                  [enable_prof_libunwind="0"])
   else
     LIBS="$LIBS $LUNWIND"