diff -urN fparser.orig/fpconfig.hh fparser.config/fpconfig.hh
--- fparser.orig/fpconfig.hh	2012-11-11 09:25:15.000000000 +0100
+++ fparser.config/fpconfig.hh	2014-06-07 16:53:15.512151116 +0200
@@ -84,5 +84,5 @@
  The second version requires that the compiler supports the alloca() function,
  which is not standard, but is faster.
  */
-//#define FP_USE_THREAD_SAFE_EVAL
+#define FP_USE_THREAD_SAFE_EVAL
 //#define FP_USE_THREAD_SAFE_EVAL_WITH_ALLOCA