Fix undefined symbols for architecture x86_64: _global, referenced from: _test_so_global in tls_so-tls_so.o. bz#347982.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@15328 a5019735-40e9-0310-863c-91ae7b9d1cf9
diff --git a/configure.ac b/configure.ac
index edc57d6..57dbfa7 100644
--- a/configure.ac
+++ b/configure.ac
@@ -2410,7 +2410,7 @@
                                [vg_cv_tls=no],
                                [vg_cv_tls=$vg_cv_linktime_tls])])])
 
-if test "$vg_cv_tls" = yes; then
+if test "$vg_cv_tls" = yes -a $is_clang != applellvm; then
 AC_DEFINE([HAVE_TLS], 1, [can use __thread to define thread-local variables])
 fi