Filters: If somebody doesn't like _Thread_local, don't fail for now, just be a little slower.

When the parallel execution comes into place, we'll likely enforce this
C11 feature. It's much simpler and also faster than pthread_[sg]etspecific().

configure.ac

@@ -115,6 +115,11 @@ if test -z "$GCC" ; then
   AC_MSG_ERROR([This program requires the GNU C Compiler.])
 fi
 
+BIRD_CHECK_THREAD_LOCAL
+if test "$bird_cv_thread_local" = yes ; then
+  AC_DEFINE([HAVE_THREAD_LOCAL], [1], [Define to 1 if _Thread_local is available])
+fi
+
 if test "$enable_pthreads" != no ; then
   BIRD_CHECK_PTHREADS