Add macro for detecting thread local storage support.

This adds an AX_TLS macro which attempts to identify if the
underlying compiler/platform supports thread local storage (TLS).
The macro comes from the autoconf macro archive.

Signed-off-by: Tom Fogal <tfogal@alumni.unh.edu>
Reviewed-by: Dan Nicholson <dbn.lists@gmail.com>

m4/ax_tls.m4

@@ -0,0 +1,74 @@
+# ===========================================================================
+#             http://www.nongnu.org/autoconf-archive/ax_tls.html
+# ===========================================================================
+#
+# SYNOPSIS
+#
+#   AX_TLS
+#
+# DESCRIPTION
+#
+#   Provides a test for the compiler support of thread local storage (TLS)
+#   extensions. Defines TLS if it is found. Currently only knows about GCC
+#   and MSVC. I think SunPro uses the same as GCC, and Borland apparently
+#   supports either.
+#
+# LICENSE
+#
+#   Copyright (c) 2008 Alan Woodland <ajw05@aber.ac.uk>
+#
+#   This program is free software: you can redistribute it and/or modify it
+#   under the terms of the GNU General Public License as published by the
+#   Free Software Foundation, either version 3 of the License, or (at your
+#   option) any later version.
+#
+#   This program is distributed in the hope that it will be useful, but
+#   WITHOUT ANY WARRANTY; without even the implied warranty of
+#   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General
+#   Public License for more details.
+#
+#   You should have received a copy of the GNU General Public License along
+#   with this program. If not, see <http://www.gnu.org/licenses/>.
+#
+#   As a special exception, the respective Autoconf Macro's copyright owner
+#   gives unlimited permission to copy, distribute and modify the configure
+#   scripts that are the output of Autoconf when processing the Macro. You
+#   need not follow the terms of the GNU General Public License when using
+#   or distributing such scripts, even though portions of the text of the
+#   Macro appear in them. The GNU General Public License (GPL) does govern
+#   all other use of the material that constitutes the Autoconf Macro.
+#
+#   This special exception to the GPL applies to versions of the Autoconf
+#   Macro released by the Autoconf Archive. When you make and distribute a
+#   modified version of the Autoconf Macro, you may extend this special
+#   exception to the GPL to apply to your modified version as well.
+
+AC_DEFUN([AX_TLS], [
+  AC_MSG_CHECKING(for thread local storage (TLS) class)
+  AC_CACHE_VAL(ac_cv_tls, [
+    ax_tls_keywords="__thread __declspec(thread) none"
+    for ax_tls_keyword in $ax_tls_keywords; do
+       case $ax_tls_keyword in
+          none) ac_cv_tls=none ; break ;;
+          *)
+             AC_TRY_COMPILE(
+                [#include <stdlib.h>
+                 static void
+                 foo(void) {
+                 static ] $ax_tls_keyword [ int bar;
+                 exit(1);
+                 }],
+                 [],
+                 [ac_cv_tls=$ax_tls_keyword ; break],
+                 ac_cv_tls=none
+             )
+       esac
+    done
+])
+
+  if test "$ac_cv_tls" != "none"; then
+    dnl AC_DEFINE([TLS], [], [If the compiler supports a TLS storage class define it to that here])
+    AC_DEFINE_UNQUOTED([TLS], $ac_cv_tls, [If the compiler supports a TLS storage class define it to that here])
+  fi
+  AC_MSG_RESULT($ac_cv_tls)
+])