m4/system_heap.m4: Use system heap by default.

m4/system_heap.m4

-dnl This macro introduces a configure flag --enable-system-heap.
-dnl If set UG takes it memory through malloc/free instead of using
-dnl the custom heap data structures in low/heaps.c.
+dnl This macro introduces a configure flag --disable-system-heap.
+dnl If set, UG uses a custom heap data structure in low/heaps.c instead
+dnl of taking its memory through malloc/free.
 dnl
 dnl Using the system heap has several advantages:
 dnl - there is no artificial upper bound to the amount of memory UG can use
@@ -8,12 +8,10 @@ dnl - valgrind may see more bugs
 dnl
 dnl The system heap may also be faster, but it may also be slower.
 dnl No way to know without measuring.
-dnl
-dnl Since this is an experimental feature it is switched off by default
 
 AC_DEFUN([UG_ENABLE_SYSTEM_HEAP],[
   AC_ARG_ENABLE(system-heap,
-    AS_HELP_STRING([--enable-system-heap],[If this is set, the operating system heap is used instead of UG's own heap data structure.]))
+    AS_HELP_STRING([--disable-system-heap],[If this is set, UG's own heap data structure is used instead of the operating system heap.], [], [enable_system_heap=yes]))
 
   AS_IF([test "x$enable_system_heap" = "xyes"],
     AC_DEFINE(UG_USE_SYSTEM_HEAP, 1, [If this is set, the operating system heap is used instead of UG's own heap data structure.]))