Default configure option for JIT

[Rik]

10/27/13

One question for the 3.8 release is what to do about JIT?  Currently it is
disabled in configure by default and one must explicitly enable it with
'--enable-jit'.  Another option would be to enable it by default at compile
time (and if the LLVM libraries are available), but disable it at runtime
by setting the default value of jit_enable to false.

The JIT code doesn't appear to cause any problems, and 'make check' passes
with jit enabled or disabled.  This would suggest making it available to
users if they want to play around with it.  On the other hand, running the
jit.tst file under valgrind produces a segfault.  Also, even when jit is
disabled, running under valgrind produces failing tests with 'make check'. 
This could all be the result of the valgrind environment or it might
indicate that the JIT is doing something odd which just doesn't happen to
occur during a normal run of 'make check'.

Are there any opinions or good arguments for one approach or another?

--Rik