Call GC_allow_register_threads() to enable parallel marking in Boehm GC

src/libexpr/eval-gc.cc

@@ -53,6 +53,9 @@ static inline void initGCReal()
 
     GC_INIT();
 
+    /* Enable parallel marking. */
+    GC_allow_register_threads();
+
     /* Register valid displacements in case we are using alignment niches
        for storing the type information. This way tagged pointers are considered
        to be valid, even when they are not aligned. */